It's hard to really answer this objectively, but I'm going to say the Kanto/Gen I starters. It was a picture of them and their evolutions, after all, that got me interested in the franchise in the first place.
After that, I would say Hoenn/Gen III for having the three final evolutions of starters that I like equally. Blaziken was amazing at first. A fire starter that isn't weak to Rock types? AWESOME! Swampert's just versatile all around, and is really fun to train. Plus, like Blaziken, it covers up for one of its primary type's weaknesses, in this case Electric. Sceptile is just cool looking, and is probably my favourite of the Grass starters. Maybe I just like dinosaur/reptilian looking Pokemon, but Sceptile was really the first Grass type that actually got me interested in training Grass types.
Despite Platinum being, in my opinion, the best version yet, I have to rank Sinnoh/Gen IV last. The only line that really interested me was the Turtwig/Grotle/Torterra line. Though the Chimchar/Monferno/Infernape line has the same advantages as Blaziken's line, I just like Blaziken more. Plus I've never really been a fan of monkey-based Pokemon. The Piplup/Prinplup/Empoleon line isn't bad per say, it just seemed, to me at least, to be an attempt to cash in on the Penguin craze that was taking off with Madagascar, March of the Penguins, and Happy Feet. The show's insistence on promoting Piplup as sort of a secondary mascot of the franchise with Pikachu has also soured me to its line. It's just been shoved down our throats to the point, much like Wesley Crusher from Star Trek: TNG, that I'm starting to dislike it just out of principal.
I can't comment on the Johto/Gen II starters, as I never played those games when they first came out, but I am looking forward to HeartGold and SoulSilver. I've always wanted a Typhlosion and Feraligatr seems pretty cool, though Meganium doesn't impress. Once I start playing the remakes and getting a feel for Gen II I may end up ranking its starters ahead of Sinnoh/Gen IV.