if you mean what Pokemon are easy to train, the easiest Pokemon to level up and stuff is your starter Pokemon. it's the same with Ruby and Sapphire, the start Pokemon always seems to level up quicker than caught Pokemon (at the beginning anyway)
hope that helped :3
*Shi ~
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Easy.
Get a Golbat and train it.
You can't find "strong Pokemon". If you want a Pokemon to be strong, you need to train it.
Torchick is a good Pokemon if you train it right anything should work
Trade one from Pokemon colosseum or get a chikorita from either Pokemon emerald or Pokemon xd gale of darkness then train it to level 16.