So this is the Teemo build I use and end up doing some kickass damage with it. Seems to do a lot more than ad/as because it works off ability power, AD, and tops out the 2.5 as. So instead of just the regular auto attack doing lots of damage, so does the blinding dart and the toxic shot. This also gives you lots of damage with his shrooms. The only thing this build doesn't give is movement speed which is why I get move quick fairly early.

For runes, I use Armor pen reds because I believe it gives the most damage.
I use Attack Speed yellows for early game kills which works really well with his passive.
For the blues, really you can use anything, I find the magic resist helps with the nukers that seem to kick teemo's ***.
Quints are flat health because of teemo's low hp all game.

Masteries are the same I use with Xin, improved ghost gives teemo that extra oomf in team fights to get in and get out while offensive talents helps both his AP and AD. I choose greed because I like that extra bit of gold to speed my build, really you could go into another point.

First I grab boots of speed, 2 Health pots and a Mana pot. I find this is the best early game so you can shoot of blinding shot at will as well as have the extra health incase you get skill shot or over extend.

On my first back I usually get berserker's greaves and start rushing my Madred's Bloodrazor. I'd get the Recurve bow first for the AS because it helps with your toxic shots. After I finish the bloodrazor I go for Malady because of the shred effect and the extra AS.

Ok so after Malady you should be about level 9-10 which to me is midgame so now it's time for the kickass items. Really they can be done in any order but I like Guinsoo's Rageblade first because of the amazing AS AP and AD it gives. If you're noticing you're having trouble with health however, switch in Hextech Gunblade for the lifesteal and spell vamp as both are gained from toxic shot. Blinding dart gives butt-tons of HP with the spell vamp and if you clear a creep wave with a shroom at full build you should go from 1 bar to full. The final item I like to get is The Nashor's Tooth which should cap out your AS and give you that sexy 20% CDR.

ALSO: You should try to get blue buff as much as possible after level 8-9. This will allow you to spam your shrooms all over the map.

I grab toxic shot first because of the AS teemo gets from runes and his passive. At level 2 grab blinding dart to harrass and stop those pesky melee carries from kicking your squishy butt. Then I grab move quick for the speed boost at 3. Maxing toxic shot first will max out your damage because the hybrid build effects the poison and his AD. Next max out blinding dart for the late game anti carry and finally take your move quick for the late game mobility. Like most other champions you want to take your ulti at 6 11 and 16.

I grab flash and ghost because teemo's so squishy and needs to be mobile in a team fight to stay alive. He's a carry so he CANNOT DIE or your team will be stuck diddling themselves against the livestealing Yi that just 2 shot you. Exhaust is another good option, ignite's good too for the early kills and the late game healing reduction. Teleport's great for mushrooms and as a quick escape mechanism if you overextend. I don't really like to take heal because it loses it's usefullness late game but since teemo's squishy it may be viable, will have to test it. Any other viable options can be listed in the comments.

This is what makes people hate Teemo. It gives you map awareness, it's great for making escape routes and it annoys people enough that they run after Teemo like he's the last person on the team 24/7. So basically where you place them is in high traffic areas such as river, dragon, buffs, tribushes etc.

So basically, stay moving or stay dead, kick *** and don't forget to drop that Hextech active. Hope you guys like the build and have fun with this lovely tiny bastard. A final little tip, don't check the bush for garen :P

Edit 1: 9/9/2011 Changed his masteries around a little bit since I don't get much crit in this build. Took 2 point our of the crit damage and put them in CDR and spell pen. Also went for the Archmage's Savvy over crit chance