I've played all four games, so I've used all three starters. My first, on White, was Tepig. It's a bit disappointing, since it doesn't learn enough really good attacks and it's a bit too slow, but I liked it pretty well and used it all the way through the E4. The second, on Black, was Oshawott. I'd say it's the best of the three overall and I like the evolutions (Samurott being four-legged doesn't bother me in the least), but it's still not that great - this is, IMO, the poorest set of starters in any of the games yet. It didn't make it to the E4 (Carracosta handled the Water duties). The third was Snivy, on Black 2, and I boxed it almost immediately. I finally took it back out and used it until it evolved to Serperior, but mostly just to do it and be done with it, and I boxed it again after that. A fast defensive Grass type just isn't terribly useful, particularly in a generation that introduced two of the best Grass types ever - Leavanny and Lilligant.
I'm currently playing White 2, and picked Tepig as the starter, so that's the one I've used twice. That's partly because, in spite of the fact that Samurott's a bit better balanced pokemon, I just like Emboar better, and partly because I'm playing Rosa, and she seems to generally be pictured with Emboar.