At this point I'm not exactly sure. I've used all three of them at least once and thoroughly enjoyed all of them. Emboar was simply a blast to use, though I felt his moveset wasn't very expansive. Serperior has a lot of potential when it comes to strategy, though I never looked into it that much so at this point I'm leaning more towards using Snivy. And then there's Samurott, who — you guessed it — is my favourite. Dat moveset, 'nuff said.
Prior to the North American release of Black and White I was contemplating using either of the starters up until the very moment in which I chose Tepig. Even though I've used all of them I'll most likely end up doing the same in Black and White 2.