Kinda? This post is assuming we're all going by the in-game logic where Mythical Pokémon don't count.
Certainly in terms of being able to get every Pokémon within the game without and internet connection and/or friends it is the easiest.
But some of those rare spawns appear to be very rare, so HGSS probably has it beat in that way, given how many of the 493 at the time are available within the game (although Safari Zone is a nightmare). As someone mentioned above, Cherubi is a nightmare to get - I got really lucky with an alpha Cherrim but haven't seen another since and I'm at almost 90 hours only spent in those first two areas of the game where they're supposedly 'easiest' to find. Still not seen a Munchlax either, just have data from syncing my shinies across all my Switch games (I like that it makes their icons always appear in the shiny form after that).
Or perhaps LGPE given that like PLA all the starters are available in the wild. Or if we're only counting regional Pokédex from back when there was a National one, BW's regional one is pretty simple. One of the regional exclusive families is available in an in-game trade, leaving you with only Gothita / Solosis, Rufflet / Vullaby, Tornadus / Thundurus, and the Mascot. That said, not having every one even able of being seen in the game sucks.