In White, I chose Snivy, and am glad I did. I used it throughout the story, and it still remains on my team. She has sentimental value, and compliments different team builds I have.
In Black, I chose Oshawott. After the story, I relegated it to the PC Box. I have too many other Pokemon I can get more use from. I also hatched a shiny Tepig in Black, but due to its nature (Modest), I'll probably leave it unevolved. If I breed a better natured Tepig, I'll see how Emboar works out.