Pros: Teemo does a high amount of damage before the enemy can reach him.
Teemo's Shrooms can kill enemies if they are low or have a low amount of health.
Teemo's a good early gamer. Best at level 9, when his E or Q is maxed.
Teemo can kill enemies even if he got killed by his poison.

Cons: Many Champions can counter him (Blitz, Shayvana, Leona, etc.) 'cause his blind doesn't work 'cause of the Next Auto Attack does extra effects.
Teemo loses Mana VERY fast after death, 'cause of placing Shrooms.
Teemo's VERY squishy against Tanks 'cause after doing a high amount of damage, Teemo will barely have any Mana left while the Tank still has a high amount of health.

Teemo's Poison adds extra damage on impact and deals damage over time. So Teemo and attack the Creeps at a higher health amount.

It matters what place you're in;
If you are playing with a Jungler that doesn't need a leash (Like Shaco), then the sequence is: Q,E,W,E,Q,R,Q,Q,Q,E,R,E,E,W,W,R,W and lastly W.
If you are playing with a Jungler that needs a leash (like Lee Sin), then the sequence is: Q,E,W,E,Q,R,Q,Q,Q,E,R,E,E,W,R,W and lastly W.
If you are playing with a Duo Top, then the sequence is the same as if you are playing with a Jungler that doesn't need a leash.

Teemo is a good Champion for when you are playing Ranked or if you are going against Easy or Duo Top laners (Trundle or Renekton, and some others). Teemo is an Okay ADC, partly because AD doesn't really work well with Teemo. Flat AP Teemo is a good choice, but you won't push your or other lanes very well by the lack of Attack Speed. Teemo support works ONLY in the early game and doesn't work very well at about level 3, because of the poison stealing the ADC's kills and creeps. Mid lane Teemo is not a very good choice as most of the Mid laners are flat AD or AP (Leblanc, Kha'Zix, etc.) and Teemo won't survive for long because of the high damage output and he doesn't have very high Armor or Magic Resist.

Teemo is mostly a Solo Champion and go for the 1v1 against the lower levels (Support or ADC, because of the lack of farming)cor the Junglers because as the creeps do damage, Teemo can kill them even if they run away by his poison. when he is in team fights, Teemo usually goes second, behind the Tank or the one that can take the most damage and deal a high damage output and sometimes KS (Kill Steal), but that's normal. When the enemy team pushes your side, Teemo usually hides around behind the lane that is being pushed and most of the times chases the lower health enemies to help the team.

Types: AP & ATK SPD = Best
Flat AP = Good
ATK SPD = Good
ADC = Okay
SUP = On the Bad Side
Lanes: Top = Best
Mid = Good
Jungle = Okay
Bot = On the Bad Side