Pros

Fast
Easy kills
Mushrooms
Evened out build
A lil' bit tanky

Cons

Only carry three mushrooms
Most likely to be targeted
Doesn't talk a lot

Well first off, Teemo is know for fast killing, Well I've got a solution for even faster killing. When you make Teemo an AP build, he is pretty OP if you ask me. I've tried this build with many different games, you will need at least 1 tank and 2 AP and 2 AD people, you can go in any of those categories. Teemo will be a bit tanky, an offtank, He will have quite a bit of HP so he can survive a few turret hits if any of his enemies are running away.

Teemos' runes can be in any form, as long as he has AD, AP and AS runes, he is good to go! Teemo is also good for making "Mini Wards", which are his mushrooms and can also be triggered alerting Teemo that someone is hiding in the bush or doing Baron or Drake.

Teemo just has full attack, mainly he needs his Magic Penetration so that his mushrooms will be more useful, this makes Teemo OP as he can easily be blahblahblah.

Teemo is gonna be evened out, when in combat, Teemo will gain his RageBlade Passive and ends up getting an extra 48 AP. This is really useful as he will have mushrooms planted around in a lot of spots. Say that an enemy is running around and ends up hitting one of Teemos' mushrooms ready to gank him in mid whilst you are killing some Minions, This will give you an Extra 48AP resulting in 400+(90% of Ability power+48) when level 16, If an enemy triggers 3-5 mushrooms in one spot, it can result as a KO without you having to do much work, The Hextech Gunblade is also useful for slowing down enemies if they're trying to get away. Of course whilst their being attacked, you'll get ganked on by their team mates but if you place your mushrooms in every exit/entrance of your lane, ganking will slow them down and have a clear escape routine.

T%he skill sequence can be in any form, as long as you get your "Toxic Shot" maxed first then "Move Quick" Last. Teemo is know for mushrooms, and being tiny. The Mushrooms take a big part in Teemos' character as it enables him to be OP, The Mushrooms are created every 15-30 seconds and it still creates them whilst you're dead.