Teemo's facts:
1. If enemies can get closer to you and use their abilities on you -> You will die
2. Early squishy, easy target
3. Lack of sustain
4. Require a decent amount of gameplays
5. Small champion
6. Cutest character in LoL
7. Has the most annoying R in the entire game (if placed right!)
8. Fun to play champion
9. Has awesome skins
10. Has only one counter! (Oracle!)
11. If you play this champ, other female player will fall like trees
12. Has various playstyle
13. TEEMO IS SATAN! Doomsday confirmed!

My personal enjoyment runepage :

RED : Magic Pen
YELLOW : Scaling Health or Flat Armor
BLUE : Scaling MR/CDR
QUINTS

18 / 12/ 0

FEROCITY :Maximize Damage Output
Good for team fights when you throw your shrooms at enemies and wait for your team to do the rest
Maximize Damage Output
Synergize well with your Shrooms + Rylai slow
MagicPen
Damage over time + your E's Passive + Liandry's Torment

CUNNING :
Teemo loves going fast
Early game sustain
SATAN HAS NO MERCY !!!
This is good (y)

When you meet heavy relying AA enemies : R > Q > W > E

When you think you can harrass enemy laner early stage : R > E > W > Q

Since it costs you 50 Gold for a health potion -> just buy Corrupting Potion for early sustaining ( plus you have your cunning points)

Finish this first asap :

Haunting Guise -> Liandry's Torment

And then Rylai second

Morellonomicon next because it lets you kill enemies with less than 35% and add Grievious Wounds effect.

Randuin's Omen is for ADCs . Its a suprise present, they wont expect it coming from smthing small like teemo

Guardian Angel because SATAN NEVER DIES !!!

Situational item : +AS Nashor's Tooth. I recommend just get the Stinger and DO NOT finish this item since you have more than enough CDR.

WITH THIS BUILD

PROS :
- Insane stats : Health , Armor , MagicPen , Damage Over Time (DoT)
- Can not be brusted
- Enemies can not do anything with your damage once they hit your shrooms, and that damage is ridiculious.
- Enemies can not excape from you when your shrooms explode + Rylai's Slow
- % Health Damage from Liadry's Torment
- You can go literally every lane you want ( top, mid, bot) except jungle.

CONS :
- Early sucks if you cannot or dont know how to kite properly
- Squishy early
- Need money to buy expensive items
- The real power spike is when you get 3 item : Liandry's Torment | Morellonomicon | Rylai which will be about >30 mins
- Vunerable to CC/Hard CC ( if you can, just juke them CC, you are very mobile ;) ! )

Teemo is indeed a fun to play champ. but to play him properly you need to play as Teemo a lot.

When in combat/team-fight , throw your shrooms directly to the enemies. and ALWAYS I REPEAT ! ALWAYS KEEP A SPARE SHROOM JUST IN CASE !!! YOU SHOULD USE YOUR SHROOMS WISELY. AND NEVER LET YOUR ULT FULL 3 STACKS, YOU NEED TO USE YOUR SHROOMS AS OFTEN AS POSSIBLE !!!