Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
1)Teemo is a good player there can keep you off the line completely negates the (pitifully small) 4% lifesteal
2)The regeneration comes from constantly attacking. 4% is tiny, and at your low damage and attack speed, this issue is magnified. This also pushes your lane faster and makes it harder to last hit (timing your animations/attack speed versus waiting for the last hits). This will also hamper your ability to farm, and NOT your opponents, as being under their tower is favorable to THEM, and not you. They can last hit as they please (certain types of minions take certain numbers of tower shots), and you cannot, as they can harass you to no end without retribution.
Speed and AP energizes with his Toxic Shot, even moreso now that they have adjusted it to do more magic damage on hit.
Poor Item Choices
Berserker's Greaves Yuck.
Trinity Force Has all of the right stats, correct? Teemo doesn't spam his abilities enough to justify the Sheen effect, or the cost of grabbing the whole item. Frozen Mallet + Zeal is better than Triforce while being ~400 gold more. Works much better on funky hybrid Teemo builds.
Lich Bane See above. More AP-oriented anyway.
My Top Two
Exhaust/Ignite and Ghost. More debuffs and damage for Teemo, plus mobility for defense and offense. Not listing Flash here because of the pending removal. Since the Cleanse nerf, I've found cleanse to be EXTREMELY situational AT BEST, and pretty much not worth taking in 90% of matches.
Other Good Combos
xxx/Flash - Currently Flash is extremely useful at the least, if not imbalanced, being an excellent defensive AND offensive spell. Flash is a great tool for finishing, juking, saving allies with a blind, ect. This is not a flash guide, so I won't list all of the intricacies associated with the spell. Suggested for the newbies who get themselves into trouble more often than not, as the game has started to shift away from flash.
Ignite/Exhaust - Very dangerous if heavy CC is present on other team. Augmented by your masteries, though, makes this combo extremely potent. Blind+Exhaust makes one-on-one fights nasty for most opponents, and the ignite packs a punch and brings additional utility. No escape aids here, though, which hurts squishy Teemo.
xxx/Teleport - With a hefty amount of mushrooms placed at strategic locations, controlling the map can lead to many gank opportunities. You can use teleport to teleport to a mushroom hidden in a bush for a silent ganking opportunity. In most cases, I do not find the added utility of teleport to be more useful than that of other combinations.
Spells I don't like:
Heal is a crutch that for MOST heroes (i.e. non-support/healing/randomness comps) isn't seen past summoner level 15.
Rally is meh, more suited for tank/support champs, if considered at all.
Fortify is meh as well, see above.
Clarity should not be ever needed.
Revive is never an option. Ever.
-Creep line control, easy farming
-Harassment of enemy champions, protection of harassable allies
-Powerful lane presence
-Lack of stun/slow
-Might draw unfavorable (unharassable) matchups
-Squishy and slow (low base move speed, move quick unreliable with many creeps/champions throwing damage around)