I started out with Tepig. Tepig is a Fighting/Fire dual type when evolved, so it was the most useful starter since it could wipe out all those Purrloins, Lillipups, Patrats, and bug types.
The second Pokemon I kept was from the Dreamyard. My Munna had good special attack, and the learnset was pretty beast. Munna learns Psybeam pretty early into the game! It can learn Nightmare, Hypnosis, Dream Eater, and Psychic later on. My advice is to NOT evolve a Munna till it learns everything. Musharna cannot learn moves if evolved too quickly.
The third Pokemon I kept was a Sewaddle I found in the inner part of Pinwheel Forest. Sewaddle is really useful against Dark, Psychic, and grass types. When I raised it to become a Leavanny, it easily became one of the most powerful Pokemon in my bunch. Even though it does have a lot of weaknesses, it is pretty damn useful.
The fourth was a Seismitoad (evolved from Tympole found on the outer part of Pinwheel Forest where the nurse trainer is). Dual water-ground type. Enough said. It's great to invest in one, there won't be a time where you would be dumb enough to use it against a grass type anyway.
The fifth was a Ducklett from the Driftveil Bridge, which I used mainly as an HM slave (Surf, Fly, Dive, etc)
The sixth I kept was a Haxorus.
And...the monkey are pretty darn useless, lmao! I would rather use my Leaf Stone on another Pokemon.