Diglett can be found with a 95% encounter rate in Diglett’s Cave. Dugtrio can be found with a 5% encounter rate in Diglett’s Cave.
Diglett and Dugtrio are an amazing duo of Pokémon who will get you through quite a few challenges, but if you take them to the endgame I can assure you they're not going to hold up nearly as well as certain other ground types (à la Rhyhorn). But it will hold that long, easily, due to its blistering speed and amazing spam move in Magnitude (and later Earthquake, if it lives until level 51 to learn it).
...Assuming you catch DIGLETT. Dugtrio actually is worse, as despite being evolved and coming at a higher level you lose the spam move in Magnitude and that hurts you a lot.
Rival (S.S. Anne): Diglett is, at this point, likely at a low level since you haven’t had much time to train it. It really isn’t good against anything that your Rival has except for Charmeleon, but that’s slightly situational as not everyone chooses Bulbasaur as a starter. Diglett can’t touch Pidgeotto, Raticate’s Hyper Fang and Kadabra’s Confusion are too strong for the extremely frail Diglett, and Wartortle and Ivysaur... well, that should be a given. Even for Charmeleon, you should probably find someone else with better defenses to take it out.
Gym #3 - Lt. Surge (Vermillion City, Electric-type): Isn't even a threat until Raichu, and it only if you haven't really touched your Diglett before the gym and caught it at level 15 or something (protip: actually RAISE it beforehand, it should be close to evolving by the time you fight Surge if not already evolved). DIGLETT DESTROYS SURGE EASILY, WE ALL KNOW THIS.
Rival (Pokémon Tower): You win against Charmeleon, are likely to win against Kadabra, will win against Growlithe although Intimidate is annoying as all get out, and if you waste the sole Rock Slide tutor in the game you beat Pidgeotto. Don't fight Gyarados, as even though it has no Water moves, Thrash and Dragon Rage are killers to you. Don't fight Exeggcute or Ivysaur, as they are bulky enough to stop you and can kill you easily. Don't fight Wartortle because it's going to win.
Giovanni (Rocket Hideout, Ground-type): You're going to win against Onix, and Rhyhorn should go down with ease. Kangaskhan can resist your attacks and OHKO you, though, so don't do it. Unfortunately this is a sign of things to come...
Gym #4 - Erika (Celadon City, Grass-type): The only purpose Dugtrio can serve in this gym is to Cut down the trees blocking the way to Erika. When it has done that, it can go hang with the pervy old man and stare at the gym girls.
Gym #5 - Koga (Fuchsia City, Poison-type): His Koffings have SelfDestruct (fun fact, they're actually identical in every way!) so Dugtrio is best left behind. Weezing is stuck doing NVE Sludges as its best attack, but SmokeScreen and Toxic will annoy you before you get the kill (which will take a while, as it has Levitate to avoid your Ground-type STAB and an insane Defense stat to avoid taking much damage from any of your coverage moves). Killing Muk is all about getting quick hits before Minimize and Acid Armors add up, and it has much stronger Sludges - you should come out on top if you don't Dig and Magnitude doesn't come up bad. You win outright against most of the trainers, though!
Fighting Dojo (Saffron City, Fighting-type): Lots of non-bulky slow things will die to Dugtrio. Hilariously, though the Karate King’s Hitmonchan doesn't fall in that category, you're still going to win - Dugtrio may have a bad Special Defense, but it's still better than Hitmonchan's Special Attack somehow, and Hitmonchan's going to spam Ice Punch.
Rival (Silph Co.): The only changes from the above Rival fight are that you only beat Charizard with Rock Slide now. Also, lolfuturesightalakazam is easy pickings.
Giovanni (Silph Co., Ground-type): He's gained Poison types, eat them alive. Kangaskhan is still a threat though, and Nidoqueen COULD cause issues if it survives a Magnitude. Dig may be a better option here.
Gym #6 - Sabrina (Saffron City, Psychic-type): You can OHKO Mr. Mime and Alakazam. That's... pretty much the extent of what you should try to do, unless you got Rock Slide (again, why?) and want to take out Venomoth. Dugtrio is one of the few Pokémon in the game that can keep pace with Alakazam, so it actually has the potential to be useful. Pray for high Magnitude rolls.
Gym #7 - Blaine (Cinnabar Island, Fire-type): Eh... you win against Ponyta and Rapidash, most likely. Growlithe is going to make you do nothing because of Intimidate, and DO NOT send it against the Extremespeed using Arcanine. Remember the fate of Doakes? You have a Surfer. Use it.
Gym #8 - Giovanni (Viridian City, Ground-type): Nidos are a maybe, Rhyhorn is a probably, and as for his Dugtrio, it has EQ and yours probably does not (if it does, then KILL EVERYTHING). Keep in mind that this gym is pretty much kill-or-be-killed thanks to Dugtrio’s frailty.
Rival (Route 22, pre-Elite Four): To reiterate: Charizard and Pidgeot go down only if Rock Slide is there, Growlithe and Alakazam go down easily enough, whereas Gyarados, Blastoise, and Venusaur win, Exeggcute probably wins, and the new wrinkle is a Rhyhorn which we have just seen and handled with Dugtrio.
PUBLIC SERVICE ANNOUNCEMENT: PLEASE DO NOT HAVE DUGTRIO IN YOUR TEAM WHEN YOU GO TO FACE THE ELITE FOUR. OH, YOU’RE GOING TO DO IT ANYWAY SINCE IT’S BEEN WITH YOU SINCE VERY EARLY ON AND IT’S PRETTY EFFECTIVE AT TIMES? WELL, I GUESS THAT’S OK. BUT DON’T SAY WE DIDN’T WARN YOU.
Elite Four Lorelei (Indigo Plateau, Ice-type): Regardless of what you're on, every one of Lorelei's Pokémon will win and kick your ass dead. Well, you may beat Jynx with Rock Slide, but even then, what the hell are you doing? STAB Earthquake has the exact same power as super-effective Rock Slide, and in a OHKO-or-die situation like this, you probably don’t want to chance the miss.
Elite Four Bruno (Indigo Plateau, Fighting-type): ONE of the Onixes may go down to Dugtrio, but at this point they can do some damage so maybe not. Be careful no matter what. And don't use Dugtrio against everything else except maybe Hitmonlee, which has a ridiculous Defense and will probably die to Earthquake.
Elite Four #3 - Agatha (Indigo Plateau, Poison-type): Arbok's the only one you have any really good chance against, unless you have Rock Slide and want to TRY against Golbat and the Ghosts. You'll probably lose.
Elite Four #4 - Lance (Indigo Plateau, Dragon-type): You can beat Aerodactyl with Rock Slide, I guess. Maybe pull a surprise KO against one of the Dragonairs. But it's better to not count on it.
Rival - Champion Battle (Indigo Plateau): See previous fight for details, note that Rhyhorn and Growlithe are now Rhydon and Arcanine which will kill you if you cannot kill them (likely). Note: Arcanine can OHKO you with Flamethrower, and its Intimidate will most likely make it so that even STAB super-effective Earthquake won’t one-shot. Remember Doakes from Fire Red: Hard Mode?
Diglett and Dugtrio have a really samey movepool, unfortunately. Because of this, only the important moves will be mentioned (so no Scratch, Growl, Sand Tomb, etc).
Dig is the namesake move, but unfortunately this was the generation gap where it has only 60 base power, so the move is pretty weak. You won't use it too much. Magnitude is only available to Digletts (no Dugtrio is able to have this move), but on average it's 70 BP (that's a Magnitude 7) and it maxes out at the insane 150 BP BEFORE STAB, meaning with its 30 PP it's your spam move (Magnitude 4, unfortunately, only has 10 BP to start with). Fury Swipes is weak, but Diglett really doesn't have too many options for Normal moves early... and after level 21, the good moves vacate until level 38 when you get Slash, a strict upgrade over Fury Swipes, and then at LEVEL 51 you finally learn Earthquake which is obviously great and all. If you somehow grind to level 64, you get Fissure, which is pretty bad (getting good accuracy on it requires you to massively outlevel the target... if you do that you can just KILL THEM, you know.) but Dugtrio doesn't have many options. Fun fact: Fissure was actually the signature move of Diglett and Dugtrio in the second generation, as other than breeding with these two and/or trade-ups with the TM, no other Pokémon could get the move.
Oh, it should be mentioned, you can get Tri Attack from the Move Relearner. It's physical and stronger than Slash, so it's cool, but if you're still using Dugtrio by Sevii you have to have some pretty massive courage anyway.
TM-wise you get... NOTHING. Well, actually, in the postgame you get Sludge Bomb which is really useful, but by that point Dugtrio is either off the team or DEAD. Rock Tomb is notable, somewhat, if you don't want to use the tutor for Rock Slide (you don't, there's better things for it like Machamp and Rhydon), but other than that we're talking standard fare stuff like Toxic and Body Slam and Double Edge that other things do better. Secret Power is a good option that comes much earlier than Slash (you can buy its TM at the Celadon Department Store), and its added effects make it arguably better than the crithax alternative. Aerial Ace is worth considering, seeing as it’s really your only way of beating Grass-types and Bug-types, and it’s hilarious to think about a mole that’s stuck in the ground using a Flying-type attack. Hyper Beam and Return are options that are pretty much available to everything, but seriously. This isn’t Red, Blue and Yellow. Hyper Beam sucks now. Got it? Good. Also, Return’s too good to be wasting on something like Dugtrio. You’ll be fine with Secret Power or Slash.
Recommended moveset: Earthquake, Tri Attack, Rock Slide, Aerial Ace
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
Diglett Line Ratings
What Nature do I want? You want Adamant. You NEED Adamant. If that is not possible, then any Attack-boosting Nature, even Brave, will do. Speed boosting Natures are unnecessary because Dugtrio is already fast as shit and it has next to nothing else worth boosting. AN ATTACK LOWERING NATURE ON DUGTRIO IS AN INSTANT "NO".
At what point in the game should I be evolved? Either right before or right after fighting Lt. Surge.
How good is the Diglett line in a Nuzlocke? Great for about three gyms, fizzles HARD after Koga. Not even being super-effective over the last two gyms saves it from its TERRIBLE stats. It’s great for the mid-game, but quite frankly, once you get toward the later stages, it sucks.