With Turtwig as the starter: Torterra, Staraptor, Gengar, Rapidash, Floatzel, Lucario
Torterra: Earthquake, Rock Slide, Razor Leaf, Rock Climb/Cut
-Your Starter. Earthquake is STAB and it is excellent at that. Rock Slide is coverage. Razor Leaf is Grass type STAB. Rock Climb is an HM attack. As is Cut. Rock Climb is the better attack but I think Cut is more useful for ingame.
Staraptor: Fly, Return, Close Combat, Aerial Ace
-Your Flying type. Needs to have Fly, which is one of the most useful utilities ingame. Aerial Ace provides more reliable STAB though. So does Return, although Return doesn't have SE coverage. Close Combat does though. Having high attack, high Speed, and the excellent Intimidate makes this thing more usable than past Flying types. Thing is BL, which is pretty much OU.
Gengar: Shadow Ball, Thunderbolt, Energy Ball, Sludge Bomb
-Even if you can't trade you can still get a Gengar if you have any of the Gen III games. You can also get one through the trade glitch. Shadow Ball and Sludge Bomb are STAB while Thunderbolt and Energy Ball are coverage. Gengar is great in that it has excellent typing (immunities are even more amazing ingame than in competitive, as many foes won't have coverage attacks) and excellent Special Attack and Speed.
Rapidash: Flamethrower, Strength, Megahorn, Iron Tail
-Pretty much the only Fire type you can get ingame, at least before he E4. It has decent offenses, and nice Speed. An immunity with Flash Fire isn't the greatest in DP ingame, but it doesn't hurt. Flamethrower is STAB. Megahorn and Iron Tail are coverage. And Strength is a helpful HM.
Floatzel: Surf, Waterfall, Crunch, Rock Smash
-Every ingame team needs a Water type, if only for the fact that Surf is the most important move in the game. Like everything else on the team, Floatzel has good offenses and good Speed. And it can be obtained early, again like most everything on the team (the entire team can be obtained before the 7th Gym. And 5/6 can be obtained before the 3rd Gym). Floatzel is like an HM slave, it has Surf, Waterfall, and Rock Smash. Crunch is coverage.
Lucario: Aura Sphere, Flash Cannon, Dark Pulse, Dragon Pulse
-Everybody wants a Lucario. An awesome Pokemon with the coveted Steel typing, plus the lovely Fighting type. Excellent offenses and solid Speed makes this Pokemon a keeper for ingame. Flash Cannon and Aura Sphere are STAB, Dark Pulse and Dragon Pulse are coverage.