Oshawott then Snivy then Tepig. I like all three but Oshawott was my first pick in BW and its evolutions pulled through for me, especially in the E4. I really love Samurott's roar, it sounds like a motorcycle and it's intimidating like a powerful battle cry. The armored samurai design is great too.
In Black 2, I picked Snivy which also proved to be a reliable and powerful starter, and it and its evolutions have that majestic look. I like Tepig too, even if people give it flak for being another fire/fighting