Not Updated For Current Season
I just wanted to post this build before I forgot. A description is to come. For the meantime....
This is a good build for playing mid. Get to the center of the lane as soon as possible and sit with stealth. Depending on the player/champion you're against you can pop out of stealth with exhaust/ignite and blinding shot for a first blood. Or you can wait for minion to kill eachother and hit level 2 for toxic then go for kill but this can be difficult because you aren't helping minions push against the champion.
Do the normal with teemo and put mushrooms in the bushes around your lane and near baron, dragon and buffs. This is good for map awareness for the team and good for team mates to run through when being chased.
With the items I personally try to get the life steal items first when buying a bloodthirster for survivability. Buy 18 if the game drags long enough you should have a little over 300 attack damage on top of the good attack speed and health for group fights. If you're doing good, most of the time you'll be a target in a team fight because teemo is squishy. Frozen mallet helps out with that a lot.