Favourite: Sceptile, I love it's design, and it's signature moved (which other Pokemon copied), Leaf Blade. Samurott, Blastoise, Feraligatr and Serperior are pretty cool too.
Least Favourite: Emboar, obviously. While not as bad as Dratini to Dragonite, he looks nothing like his 1st form in any way except in colour scheme. Then there's the fact he's the third Fire/Fighting type in a row! Masuda said that they put most of their effort into the starters but Emboar (and Infernape, and Quilladin...) is evidence of their laziness. I always thought Venusaur and Meganium were a bit plain as well.