1st and 2nd generations only. Ain't nobody got time for that. --- I wish Blastoise was my dad. He wouldn't beat my mom like what her boyfriend, Johnny does. If Blastoise was my dad things would be different around here.
Only W2, but if you count spin-offs I've done it in a ton more games. Pretty sure I've completed Ranger's dex 3, maybe 4 times and Shadows of Almia's at least twice. Guardian Signs once. The original Mystery Dungeon's dex once. Probably more but I forgot. --- Official Zangoose of the Pokemon X boards 3DS FC: 3609-1237-6725
Did it legitimately in Blue and/or Yellow, except Mew was hacked. I still got the diploma without it though, so it counts.
Did it in Silver with some cheating involved. Since my previous games had their memory erased somehow (and I think I lost them for a while too), I had to resort to gameshark for the Pokemon that were exclusive to R/B/Y, (The starters, fossils, and legendaries), as well as Mew/Celebi.
Never did it in Gen 3 except in a couple hacks that included every Pokemon in one game. And actually, with both of those, I still had to use a code for a couple Pokemon, since the hack creator left a few out. I never bothered on the physical copies because it required too much trading, and I didn't actually pick up any of the games other than Sapphire until after Gen 4 was out. I almost had a complete regional dex in Sapphire, but I was still missing about 5.
In Gen 4, I did it in Pearl with a boatload of self-imposed restrictions, the major one being that I wouldn't trade with anyone, and I wouldn't even trade with myself unless there was no other option. So I am proud to have a complete dex of all 493 with only 4 trades registered on the trainer card. Definitely my best accomplishment in the series. I also have a pretty decent collection in SoulSilver (everything available within the one game itself)
And finally in Gen 5, I don't have a complete collection yet. My restriction this time is that I wouldn't accept any Pokemon imported from older games (or traded from anyone else who has done that), so there's still no way to access the older starters (aside from some events), the Pikachu family, Unown, and some legendaries. Now with Gen 6 coming out soon, I probably won't be able to complete it. But at least I got as close as possible. I might eventually give in and import what I need in a few months. --- If you actively hate Justin Bieber... then you don't know what "intended audience" means. Hate Twilight instead, it's far worse.