I like on-hit teemo. for major items i go: Wits-Frozen mallet-Bloodrazors-Malady
once u have all of that each aa of yours will:
deal toxic shot DoT, toxic shot dmg, 42 dmg from wits, a slow, a % of their health, and reduce their magic resist so that your next attack does all that but stronger.

There are two meta Teemo builds: Fighter Teemo, and AP Teemo.

Fighter Build:
Max: E, Q, W
Start with: Boots, HP, then grab a Phage (for early game sustain, slow, and damage), you can head for boots here (Merc Treads / Ninja Tabi / Zerk Greaves) or proceed right to Wit's End. Then get your Frozen Mallet, and you'll be all set for team fights (additional items are your choice; for more damage get Madreds Bloodrazor, for more sustain get Hextech Gunblade, etc)

AP Build, aka "The Mushroom God"
Max: Q, E, W
Grab boots and hp (as always) and proceed to Catalyst, get Sorc Shoes, get RoA, then either Void Staff or Rabadons (depending on how much gold you have). You will be heavily criticized for this build, but it is actually EXTREMELY effective (if you have good ult placement).