Ya know, I'd have to say Gen 3 for me. The Treeko line was very interesting to me, a speedy grass starter, the Mudkip line was just interesting, and the Torchic line first introduced us to the Fire/Fighting combination.
Which, unfortunately, every fire starter since has copied. But I digress.
My second place favorite would have to go to Gen 2, since all the starters there respected their element types, and said *bleep* you to getting a second element tacked on. The only generation to have all its starters be single-elemental.
Gen 1 would have to be in third, it's the original, it deserves some respect, Gen 4 and Gen 5 are kinda tied for me. --- --- Swooping... is... BAD!
Gen 1 and 4, but don't make me pick between the two please :P --- If you've no respect for the fact that people have put the time aside to play a game with you, play a single-player game. There are thousands to choose from.