I voted for Gen IV just because I've never entertained the thought of using Chimchar or Turtwig, ever.
Gen V is a hugely close second though, because battle-wise they're all horrible. Aesthetically though, I like using them all. To me they're a jack of all trades, master of none bunch ^^;
Gen I is mediocre to me, I like them all but they got a bit old.
Gen II is pretty good to me, I only use Chikorita but I've used the others in the past and enjoyed all play throughs with them.
Gen III is my absolute favorite. They have the best starters, Sceptile is pretty bad but I still like using it once in a while, and Blaziken and Swampert are just plain beastly.