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If we're talking about being dropped right into the Pokemon world, then Bellsprout or Sableye. They're tied for my absolute favorite Pokemon. Bellsprout I love just because he's this little wispy little guy, but he could probably do some amazing tricks by using his vines to move around. And as the anime showed, he can be agile as hell. Sableye is great just for being a sneaky little imp, and his design is great.
But if you told me I was going to be dropped into the wilderness somewhere with any starter Pokemon, I'd have to go with Machop. Having an extra set of hands (and opposable thumbs), but with like 20x my own physical strength? That's the sort of friend you'd want in a tough survival scenario. And if shit ever got real and your life was in peril, you could take off the Power-Saver Belt and let it annihilate anything between you and safety.
Plus, I think having a humanoid Pokemon would be pretty fun. It'd be less like owning a pet and more like traveling with your best buddy.