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.
I picked Snivy. I had picked Tepig in my White and simply boxed it because I never had any intentions on using it. On Black 2 though I did use my starter so I chose the starter with the design I liked best. It was pretty alright. Obviously not the best starter but it did well and I didn't become frustrated with using it or anything.
When I first started Black 2, I went with Tepig because I didn't really use that one before. But then I restarted the game with a planned team, and Magby was on my team. I chose Oshawott for my current game, since--as I said--I already have a Fire type and a Grass type.
I prefer, Oshawott due to his evolution into Samurott. However, I started out with Snivy. My reason being, "Rosa". I'm well aware that Pokemon typically base names around the Pokemon type of said character. Examples: Misty - Water Type, Gadenia - Grass Type, etc. and this left me feeling obligated to start with Snivy.
I actually picked all 3 (multiple games), Snivey's evolution would be used to catch pokemon cause grass pokemon are usually the best type to take your time with and attack carefully or switch out, Tepig.. Was more like a tank, he would get slower through out the evolutions and his defence and attack ↕ making emboar a wall for my team so i could go through lots of battles barely healing him, while oshowatts evolution.. I love water pokemon, water pokemon where mainly defensive except for the rare few like gyarados and kyogre ect. samurrot was actually strong, i nearly cleared the Elite 4 with just aqua jet untill i hit alder, then i had to use my dreaded leavanny x.x
Oshawott, and it has always been Oshawott thus far due to my tendency to pick Water Starters. The best thing is that whatever the nature it has a good movepool. It also makes a good HM Slave as Samurott can learn 5 HM moves.
Boilurn, the Scald Pokemon and the evolved form of Hottle. It can burn the holder by the lightest of touches when it boils. It can boil 3 gallons of water in one minute.
Mine was Snivy, and as much as I love the Snivy line it was a mistake as choosing it as my starter. It had a bad nature, and I didn't realize this until I was far into the game, and just really lacked any power. Wouldn't have been so bad, but I was doing a Solo-Challenge and that mad it really tough especially when fighting Burgh.