Great question 8D I love many of the middle evos of the starters, and shall post several votes even though we can only pick one in the poll.
Charmeleon is my obvious favorite. It is sleek and has lost the innocent looks of Charmander but still doesn't look as dorky as Charizard (no offense, Charizard lovers!)
In the same way, Wartortle is a perfect in-between for the cute Squirtle and the bulky looking Blastoise. Loved it in the anime, also.
I was happily surprised when my Torchic evolved in Ruby. Fire/Fighting! In Hoenn, it was still cool and not overdone like in Unova. Combusken has a rather perfect look, imo. Again, it's the perfect link between the cuddly basic pokémon and badass final evo Blaziken<3
I guess Grovyle deserves a mention, just because I really liked that it wasn't four-legged, and its leaf blades were really cool. Never used it much in the games though.
Okay, this is probably my second favorite. Monferno is cool on its own, much better than Chimchar and not as overdone as I can feel Infernape being. Absolutely love its looks, and again its typing because a fire monkey fits well as a fighter somehow.
Third favorite, probably. Dewott was definitely an improvement from the rather weird looking Oshawott (again, no offense to those who love. I just compare it with Piplup too much and I like the penguin more!) and still not as strangely complicated as the four-legged Samurott. Dewott looks amazing with its two shells and feisty stare.
All in all, I guess my favorites are those that aren't trumphed by their prevos or evos, Charmeleon & Monferno & Dewott. While others can be really cool but have even better evos or prevos (Bulbasaur, Blaziken, Empoleon). :3