All of them are good, so I'll just describe them in detail, for you to decide which one you like best.
Turtwig
Grass
Evos: Grotle-Grass
Torterra-Grass/Ground
Turtwig has a pretty low speed stat, but makes up for it with good offense and decent defense. It's only HUGE weakness is Ice, since Torterra is double weak to it. Some of it's most notable moves are Curse and Earthquake. Curse is learned sometime after the first evolution. It boosts it's Attack and Defense while lowering it's speed, which is already low, so that doesn't really matter much. Earthquake is a Physical, STAB, 100 Power move that it learns as soon as it evolves into Torterra at level 32. In coordination with Curse, this move can become VERY powerful. It's highest Base Power move is Leaf Storm, but it's special, and will drop sp.atk, so I reccomend Wood Hammer, which is learned with a Heart Scale after evolution. It will work awesome in coordination with Curse. The same goes for Crunch.
As a final movset, I reccomend Curse, Earthquake, Wood Hammer and Crunch
Chimchar-Fire
Evos: Monferno-Fire/Fighting
Infernape-Fire/Fighting
My personal favorite, Chimchar and it's evos have very low defense stats but make up for it with amazing attacking and speed stats. It has no double weaknesses, but doesn't have many resistances either. It's main threat is Psychic, because it doesn't have very much that can counter them, as far as supereffectiveness goes, and they can sometimes outspeed Infernape. The same goes for Water, except it may be able to outspeed Water types and KO them before they attack. Some moves of Interest are Flareblitz and Close Combat. These 120 Power STAB moves are quite good on him, however if your Infernape has Higher SpAtk (which it very well may) Fire Blast and Focus Blast can be taught via TM, both of which can be bought at the Vielstone department store. Close Comat will lower your Defense stats, which won't really make a huge difference, as low as they are already. FlareBlitz will deal quite alot of Recoil Damage, and Fire Blast and Focus Blast both have fairly low accuracy. Some other good moves for it to know are Stone Edge, Earthquake, and Grass Knot. Stone Edge can Counter Flying types, and Earthquake can counter most of what Infernape's other moves aren't super effective against for atleast Neutral damage. Grass knot can counter Ground and Water types, especially if they are heavy.
As a few good movesets: Close Combat/Focus Blast, FlareBlitz/Fire Blast, Stone Edge, Grass knot
Fire Blast, Focus Blast, Grass Knot, Nasty Plot (can be learned by Chimchar at level 23)
Piplup-Water
Evos-Prinplup-Water
Empoleon-Water/Steel
Empoleon has pretty low speed and not so great attack, but makes up for it with good defenses and Decent spatk. It also has no double weaknesses, and in fact has alot of resistances. It's main weaknesses are Fighting and Ground, plus Physical Electric moves like Volt Tackle (or is that the only one?). It doesn't have a great movepool, the most notable move being Hydro Pump, which has pretty low accuracy, i prefer surf there, because it still has 142.5 Bae Power with STAB. Drill Peck may seem like a good move, but due to it's low attack and speed, also considering the only weakness it counters is Fighting, fighting types will likely be able to take a hit from it and most of them will also more than likely attack first, KOing you (although you do have a chance if they used a spatk like Focus Blast, however, I don't think I've ever seen an Ingame trainer packing Focus Blast or Vacuum Wave), so Drill Peck is out of the question.
For a Final Moveset, I reccomend Surf, Flash Cannon, Ice Beam, and the last move is really up to you.