The general consensus seems to be that Second Gen is best! And I'd have to agree. To me, in addition to them being great games, the artistry of the Gameboy Color sprites is phenomenal. They're like, max four colors per pallet, which is astounding to me, since they made it look so fantastic.
But, in my opinion, beyond maybe some iffy, redundant/extraneous new Pokemon designs, the gameplay mechanics have been doing naught but improving since the beginning; Fourth Gen's battle system, with the split up Physical and Special type moves, loads more moves and abilities and so forth is just great.
If you go to Bulbapedia or something, and compare old Pokemons' old movesets to their movesets in Fourth Gen, they're more filled out with a lot more fun options and possibilities in Fourth Gen.
I know it'll be like five years but I can't wait to see what they do for Fifth Gen. I wonder if they've got anything planned at this juncture? I haven't even heard whispers of a long lost triplet to our already existing Fourth Gen twins showing its face... Also, here's to a remake of Gold and Silver in the new style (they'd be dumb not to make it, honestly)!