Sentret should be low, it has terrible power and even after it evolves it is still pretty bad. Although I guess it has some use as a HM slave of sorts if needed.
I'd like to throw up Miltank for high.
Name: Miltank- Tier High
Availability: Right after getting to Ecruteak. Only 3 gyms down by that point, and Miltank starts off with Stomp (at level 13).
Stats: Good Defence backed up by good Speed and decent bulk in general. Attack is a little lacking, but you still hit pretty hard.
Typing: Normal. not a lot you can say here really.
Movepool: Stomp initially is great for the flinch chances, Body Slam for Paralysis support, Zen Headbutt lets you hit fighting types (if you decide to stay in). Flinch moves are good given her great speed.
Major Battles: As long as you have Scrappy, you pretty much demolish Morty. Obviously looses badly in Chucks Gym, but might be able to spring some flinch hax/take a hit before failing totally. Jasmine again is off limits too. Past that however, and you're free to hit anything you like. With Thick Fat (And without) you deal solid damage against Pryce's super weak Pokémon and she can hold her own against Clair. She holds herself in the Elite 4 as well (Minus Bruno). Mine ended up with something like Body Slam, Strength, Zen Headbutt and Rollout I think and it was solid. Normal typing is good in general ingame.
Additional Comments: Definitely go with Scrappy if you can. Thick Fat may be of a bit of use later against Pryce and then possible Lance (Those Dragonites placing the random Blizzard/Fire Blast). Slow to level but I doubt most would notice the difference.
I'm on a mission to train every Pokemon Family during a playthrough of 3rd to 5th Gen! Currently at: 532/602!
Current Team (White 2 [6 badges]): Ninetales/Zoroark/Mr Mime/Cherrim/Sawsbuck/Swampert
Current Team (Black 2): Solrock/Emboar/Sunflora/Crobat/Dunsparce/Azumarill