This took me ages to type.
Turtwig gets a same-face moveset in the earlier levels, and some nice stats, though you'll be out-sped by everything unless your ~2-3 levels higher than it. Grotle gets nothing really special, just some stat boosts and Bite for more move diversity. Torterra is where it really gets fun. He tanks like a mother and doesn't afraid of anything. It gets some pretty good moves (who said EQ is bad in a Doubles? a) the game doesn't even focus on double battles and b) just send in some **** like Staraptor), though for lack of a better grass move it makes up for in the stat boosters like Rock Polish and Curse. Might as well not even focus on speed with this beast here, so slap Curse on it with some Physical moves and your moveset.
Piplup's a pretty cool guy (it's a cute penguin, you know you love him). He pretty much is going to be Bubbling the not so good out of everything early-game, losing a little spotlight around the second gym. It's got Turtwig's problem of little speed, but really it's pretty balanced stat-wise. Prinplup it'll upgrade little, Bubble to Bubblebeam to Brine, but as with everything once it gets to its final stage it's gonna be one badass motha. He gets some sweet boosts in stats, pretty cool moves. Don't like its Speed? Slap Aqua Jet on it. Sure it gets some bad weaknesses, mainly Ground and Fighting, with neutral Fire, but dat Poison resistance might save your ass a few times. Give it the right moves and train it well and you won't have a problem here, besides early game but that phase quickly ends once a team fills out.
Chimchar's pretty much opposite with Turtwig's line. It focuses on fainting to get the win out of there. It's gonna outspeed pretty much everything at a decent level, it's great at attacking Physically and Specially, but it's major drawback is it can't take hit bigger than a Bubble. You'll want to put a variety of attacking moves on it so if it gets into a sticky type disadvantage situation it can get out of it standing. A pretty cool thing about it is if you can time it's damage right it could take the hit and finish things off with the ability, making its ability somewhat useful if not risky. You'll be dominating early-game if you evolve it into Monferno.
Hope I helped!