Gen 2. Gen 1 was bad but you have to put it into perspective. GF were noobs. Plus FRLG were very good and "fixed" it. Basically it was retconned out of existence.
HGSS didn't fix GSC. Johto was still puny and bland. I thought they would make it bigger since DS holds a lot more data. But no. It was such a letdown after Sinnoh, which seemed huge. They didn't fix the levelling problems either.
Gen 4 for me and Gen 2 as a close second. Idk, something just doesnt feel right about Gen 4 for me, the whole game experience felt like a chore. As for Gen 2, I was really hoping that HG/SS would fix the main issue with the game, which was each route had just about nothing to offer and the levels of those pokemon were close to useless and always 10-15 levels under your team. It was a gen 4 based remake, why did they have to lock all the gen 3+ stuff away until after the E4, the game was fun but it's so hard to actualy build a different team each time you play it, since there are almost no choices. Why was it necesarry to throw ratatas into every single rouge GF! WHY?!?!?! Still wish they would bring back the pokemon follows you gimmik, was pretty kewl.