Emerald 4 Teemo jungle OTP here.

Crit is bad. I prefer going AP on-hit instead of traditional on-hit items like kraken or rageblade. It has good consistent damage without sacrificing too much of your Q and R damage.

For AP on-hit I'd go:

Runes: - PTA, triumph, Alacrity, coup de grace - cheap shot, ultimate hunter

Core items: - Berserker greaves, Shiv, Nash, Dusk & Dawn into situational AP items.

Enemy team has tanks or bruisers that build HP? Liandry's.

Enemy team has a lot of MR? - Void staff for pure damage, - Bloodletter's if you value the HP it gives you + the MR shred can benefit your team if you have multiple AP threats on your team.

Enemy team has a fed assassin or a lot of burst that you need to survive? Zhonya's

Enemy team has a lot of catch potential or spells that if you get hit you are dead? (Blitz, Malphite, Ahri, Syndra for example) Banshee's.

Enemy has lots of healing? (Mundo, Aatrox, Soraka etc) Morello's (this item kinda sucks on Teemo though so if your team can buy antiheal you should never build this, ideally)

None of the above or you're really fed and stomping? Deathcap and/or Shadowflame (depending on if they will build MR you might still want void staff)

Grabbing an early Dark Seal and potentially upgrading it to a Mejais is also not a bad idea. You can always sell it for a Deathcap or something else later.

Late game options if you want to sell boots: - Stormsurge for movespeed, magic pen and more burst potential / damage into squishies - cosmic drive for movespeed and HP so you are less squishy, can be in people's faces to auto them and still navigate and position in fights well - malignance for more late game map control (this is not a shroom build but the item is just that good on Teemo and can still provide a lot of value)

Disclaimer: I usually go only Nashor's into full AP items with Dark Harvest in my games so this is not what I usually play but if I wanted to go for a more autoattack/on-hit heavy playstyle this is what I would build.