It seems like every pokemon with arms can learn thunderpunch via move tutor.
Hidden power(rock)/rock slide-being a good mixed sweeper, Treecko can work with either one. But his special attack is better...
Leaf storm/Energy ball-I like Leaf storm on Treecko when his overgrow activates. He'll be switching a lot anyway, so not much worry about the stat drops.
Dragonbreath/Mud Slap-Mud slap can hurt poison types and block their attacks, Dragonbreath does an okay amount of damage, and leaves the chance to paralyze.
synthesis/leech seed-a good recovery choice.
Rock slide-Covers most of his weaknesses
Swords dance-powers him like crazy.
seed bomb-A STAB grass move powered by SD (I'd sure hate for overgrow to be active)
Aerial ace/Crunch/Crush claw/Endeavor- A good last move to give treecko extra type coverage.
Or the seldom seen Staller/Sub seeder:
Leech seed-saps foe's health
Substitute-Protects treecko, but gets health from leech seed, is good because treecko and his evolution chain are among the fastest subseeders in the game.
Protect/toxic-Instant stalling to be a jerk, or toxic for added fun.
energy ball/Giga drain-Deal with taunters, Energy ball has higher power, but giga drain can help the strategy even more in that it restores Treecko's health.