Well, I completed my first run with Snivy, now starting one with Oshawott. Here's where my opinion stands.
Snivy - I was really hoping it would be better lol. Leaf Tornado has shit PP, Snivy has few moves outside of Grass ones till late in the game and its offenses are overall medicore. I felt like it was holding me back the entire game offensively. Not to mention there are better grass-types in the game.
However, Growth / Leaf Blade / Dragon Tail / Grass Knot was sweet endgame, and his tanking ability was helpful during the final fights. Servine pretty much soloed Clay's gym. Overall, I'd say Snivy is middle tier. I'll give him points for being a starter and decent lategame, but there are better grass-types.
Tirtouga - Slow, but Solid Rock is amazing. Surprisingly good special attack combined with high attack makes it a good surf user. Soloed the Ice Gym and helped with the Flying Gym. Defensively, its a monster. It can take hits while you revive or heal your other Pokemon easily. Also, highly recommended with Quick Claw.
It's faults are pretty annoying ones, though. Rock Slide comes really late and that Special Defense isn't to great. The worst thing about Tirtouga is you either get it or Archen. Archen is pretty hard to pass up. High, would be top but Archen is just too damn good.
Elgyem - Special Attack is great, but very slow. Disappointing actually. Munna and Solosis are way better for a Psychic-type. Elgyem is too slow with too little benefit. It learns Charge Beam, but that's its only real advantage. Munna and Solosis are both bulkier, and Solosis can be immune to status damage thanks to Magic Guard. Lower Mid.
Blitzle - Surprised me, it wasn't as bad as I thought it would be. Very fast and Shock Wave early is great. Flame Charge helped me out with the Bug Gym and Ice Gym. Evolves fairly early (compared to other mons in B/W) and LightningRod is a sweet bonus.
Problems? Well, its frail and its movepool is average. The worst thing is its pretty much useless past the final gym. Galvantula is helpful against the Elite Four and final battles. Zebstrika is...not. Mid, it's great midgame mon, but terrible lategame.
Sandile - Sandile is the best Dark-type in the game, hands down. Some might argue Scraggy, but Sandile is faster and evolves quicker. Great coverage between STABs alone. Crunch and Dig allows it to destroy most of the Electric Gym, but isn't useful against most of the other gyms. Krookodile destroys two of the Elite Four. The real great thing about Krookodile however is Moxie. Moxie is ridiculous ingame.
It's only downsides are that Earthquake comes late and it isn't exactly bulky. But that's pretty much it. Top tier.
Venipede - My MVP. The king. The god amongst insects. After it learns Poison Tail at level 19, it becomes amazing. It isn't too useful against any gyms, but it evolves into Scolipede at level 30. Then you can teach it Poison Jab, Bulldoze and X-Scissor via TM and Megahorn via Heart Scale. Megahorn at level 30 is devastating. Scolipede really shined lategame, where helped out three of the four Elite Four (swept one of them) and took out two thirds of the final battles team with Megahorn alone. Being immune to poisoning is always nice.
It's problems are few. It's weaker compared to Leavanny and Galvantula, but Megahorn is so powerful its hard to notice. It does have an awkward period of uselessness between Lenora and Burgh though if you picked Snivy. High tier. This bloke carried me much of the game.