Gen 5 because
Gen 1: They were the originals, all three were useful in ways, and people will compare others to them.
Gen 2: Chickorita was a little lame battle wise I agree. Cyndaquil was a great battler and his design was cool. Totodile is my favourite though, a good design and a good pokemon.
Gen 3: Three great pokemon here, though i would say Treecko/Grovyle/Sceptile is weakest here due solely to a high sp.atk but limited powerful special moves.
Gen 4: Probably my favourite group after gen 1 (I'm not a genwunner btw) the designs done it for me, you could clearly see what each was supposed to be, they also happen to include my favourite pokemon.
Gen 5: I got pretty excited at the first glance at Tepig and Oshawott, they looked cool, both good pokemon who had more potential (i like Samurott), Emboar could have been better and could have been dark type as well as fire!
Snivy... Its a green cyndaquil with big eyes. Meh. Then someone tells me its a snake!! Serperior could be a hell of alot better!!!! Its frail =(