What Pokémon game do you think it's the easiest to complete the National/Regional dex in? I find it's pretty easy in all the Generation 4 games, but especially Diamond/Pearl/Platinum.
In these games you can:
-Get Pokémon via the dual slot in your DS.
-Swarms (Daily)
-Migration from R/S/E/FR/LG.
-Trading with HG/SS, as well as with eachother, with relative ease (Just go to the Union Room.)
-Exploit a DNS trick to get event Pokémon you normally cannot obtain.
-Breed
-Use a Poke Radar to get secret Pokémon to appear.
Not to mention you can pretty much get EVERY Pokémon, except starters, Tropius, Tangela, legendaries and mythicals, completely on your own without trading or migrating, too, in Diamond/Pearl. Platinum added some more legendaries you can get, and Tropius and Tangela, as well. So, I deem Generation IV the easiest generation, at least for me.
this is under the assumption have access to all of this, though. that requires having each gba title, a copy of both HG and SS (for their various exclusives if you can't get them through gba titles), and a second system to trade with. for the casual player, i don't think that's entirely feasible. so like it's definitely possible, but it still, to me, seems like a lot of hoops to jump through.
i'm going to exclude gen I and LGPE from this bc there are significantly less pokemon in these games since it's only kanto (+ some metal friends in LGPE) and cast my vote to the generation VI games, especially oras. there's so much available to you in oras after you get access to mirage points and such. not to mention having a wifi connection makes it all the more easy to get those pokemon. say what you want about the gts, but it's pretty easy to nab what you want if you just put up popular first stage pokemon like starters and request other first stage pokemon in return.
in this same stretch, gen VII games are easy to complete the dex in, too. while there are more pokemon than in previous games, i feel like you have to go through less mess to obtain those pokemon. you're not required to have extra games or an extra device for them. you can potentially complete the dex just with one game and device by making use of the online features, especially if you have an online trade community like we have here.
if we're excluding games with wifi connections though, then naturally gen I wins without question. less pokemon to obtain and if you don't have an extra game, link cable, and gameboy, you can still make easy exploits in using glitches.