My least favorite has to be Squirtle, even though I'm not much of a fan of Tepig either. Squirtle's design is just really, really boring and so bland, it just feels so uncreative and boring, and I really don't like any of its evolutions either. The Gen 1 starters overall are probably the most boring starter set out of all really, even though I like Bulbasaur and Charmander quite a bit. When it comes to Tepig I have as of recently slowly started to warm up to it and did take quite a liking to it due to its character in the anime, though I'm still not much of a fan of it mainly because of its evolutions. Pignite's cute, but Emboar is such a disappointment, and the fact that it's another Fire/Fighting-type doesn't help at all.
Picking a favorite starter is so much harder, omg, there are just so many that I absolutely love. Totodile, Treecko, Chimchar and Snivy are all amazing and probably my favorite ones with Chimchar being the one closest to my heart. However, I must say that Gen VI introduced three really impressive starters and I really love all three of them, and out of them Fennekin (which ironically was the one I liked the least once they were revealed) is the one whom I've grown so very attached to, and dare I say that its evolutions probably are my favorite starter evolutions as of yet. So, I'd say that it's a tie between Fennekin and Chimchar. When taking their evolutions into account however Fennekin definitely takes the price.