Generation III, but only really FRLG
I, to be honest, liked the Gen 1 pokemon most, not just because they were first, but because they were interesting and clearly weren't just the sound of the developers scraping the bottom of a metaphorical barrel (Bidoof, I'm looking at you)
However, the Gen 1 graphics were horrendous. I'm that someone is going to reply going "OMG U LIEK GRAPHIX NUT GAMPLAI U IS SHALOW", but I just like to have the sprites at least be correct. Look at the Koffing sprite for R/B, for example. And the backsprites...oh dear.
Gen 2, while great, was not the OH MY GOD SUPER MEGA AWESOME JOHTO REGION that everyone seems to think it is, but It does get kudos for having decent graphics and a gamplay that is probably the best of the series, though not by much.
Gen 3 (Hoenn) was like the opposite of Johto - the grahics were great but the gameplay wasn't amazing if I'm honest and some of the new Pokemon just didn't gel well imho.
TLDR version : Gen 1 & 2 were too bad graphically, Gen 3 & 4 have not great gameplay.
Plus, FRLG has nostalgia :D