Diglett is available in all Kanto- and Johto-based games as a guaranteed Diglett's Cave catch, as well as having a 4% chance of being found on Route 48 in HeartGold and SoulSilver. However, in Gold, Silver and Crystal it's postgame. In ORAS, it can be found with the DexNav. It is additionally available in Kalos and Alola-based games.
Completed analyses of the Diglett line so far: