League of Legends (LoL) Question: TEEMO ARGH

Adaptive Helm is good against him if you play tank

you should use Oracle Lens to deal with the mushrooms too

Teemo can a pretty big lane bully with poke that can be hard to punish, especially if you're playing autoattack-based melee champs because of his Q blind. However, he does have a relatively small aa RANGE (smaller than Darius's E), and unless he gets fed he might be annoying late-game with his mushrooms, but not all that threatening. Remember that since he really only has 2 abilities until 6 and 1 important ability (W is a movespeed steroid and E is his passive poison on aa's), if he ever wastes his Q to farm he's very vulnerable.

In terms of champs to play into him, Darius can be pretty good due to his E outranging Teemo's auto, but since Darius can be reliant on his aa's to proc his passive fully, I would reccomend practicing the matchup before couterpicking a Teemo with Darius in ranked :P. An off-meta pick that I've tried out against Teemo with some success is Swain top. Although Swain can be a little squishy until he starts building up passive stacks, he VASTLY outranges Teemo with his E and he doesn't really care about autoattacking since E1 + yank + E2 or E1 + E2 + Q procs Electrocute (I always forget if his yank counts as an electrocute attack). Mordekaiser can also mess Teemo up pretty easily with his E+Q and, since Morde is usually okay with farming up till his mid-game Rylais/Liandry's spike, he is hard to bully out of lane.

Edit: Per the other reply, rushing Spectre's Cowl into Adaptive Helm is usually a reasonable first item for tanks vs Teemo. Adaptive Helm reduces the amount of damage that DoT magic effects do, so it's good against Teemo, Brand, Cassio, etc.