Teemo is the Perfect Spy and for a Spy you need Move-Speed thats the Reason why i take Move-Speed Quints and Boots of Swiftness.
Teemo is a tiny Champ and is 1 of the best champs to get no focus.

I play Teemo as a Ap-Atack Speed Hero so i get Magic Pen and Ability Power Runes
Move-Speed Quints are very good for Teemo because at the late game you got near 520 runspeed.

For Masterys i take 9/0/21 because the 21 gives extra Runspeed and mana reg is the best because i dont buy any Mana Reg items

My Corebuild on Teemo is:
Boots of Swiftness, Because run speed is great by teemo because you are squshy and needs to run away fast
Malady, Because Extra Damage,Attack Speed and AP
Madreds Bloodrazer,Because you can go as tiny Anti-Tank and good Dmg with attack speed,
Rylai´s Cristal Scepter or Frozen Malett, Because Slow with skills and Auto Atacks

Good Summoners:
Ghost,Because you run faster then every other Champ
Exhaust,If you wanna have a kill you can do with Attack Speed good dmg in this Seconds, its a good Tactic to come out of hard Dmg Output
Teleport: Teleport to the Shrooms is nice Assasins Way
Optional:
Ignite: You got the Poison and you dont need Ignite, but its ok to kill any Healer ,
Flash: Good to come fast out of Focus
Clearse: If you are in CC(Croud Control) you can die very fast
No Go:
Cv: Why with the best Scout?
Clarity: No need because never OOM
Fortify: Let the Tank´s get it
Rally: No need, you are never in the Range
Revive: You are so fast you dont need this
Smite: Teemo Jungle, NO

Care In Early Game, start to fight after Malady not earlyer
Stay every Time behind the Tanks and focus the AD Carry´s with Blinding Dart.

+ Good Scout
+ Shrooms are good to spy enemy Buffs
+ Very fast to Come out of the Fight
+ Nice Magical Dmg
+ Cheap Items
- Squishy

Only Lasthit the Minions or you will gang fast and you are to Squishy for this.
Try to get High Farm with the 2 Philosophers you can Stay long Time on a Lane.

Focus The AD Carry´s with Blinding Dart to Save Teammates for a Xin Zhao,Master Yi or anyone other