Teemo on support can be very rewarding with denying the enemy ADC CS by blinding them, oppressing melee enemy supports with poke, and having a lot of map pressure with his traps.

Glacial Augment is great for getting out of trades, roaming and ganking, chasing enemies down, and applying Font of Life so that allies can benefit from Ardent Censer without relying on your ultimate ability or buying other items. RIP In Pieces Spooky Ghosts.

Free Boots or Stopwatch is preference, but either can be taken.

I feel like Biscuit Delivery is the strongest here, because they either help you apply pressure in lane or you can sell them and get 30 gold and then 40 flat mana which is also worth 56 gold in stats.

Approach Velocity is great with Glacial Augment for sticking to targets and running down enemies that step on a Noxious Trap.

Font of Life helps sustain your ADC in laning phase, and applies Ardent Censer when you obtain it.

Second Wind is more reliable than any other options for early game trading versus mixed damage, but Iron Skin and Mirror Shell are pretty good with biscuits and potions too depending on the matchup and can be more important situationally. For example if their support is Zyra, and ADC is Kog'maw or Corki or has magic poke like Scorch, AND their jungler does magic damage you will want Mirror Shell.
Overgrowth or Conditioning are also viable if you are confident in your matchup i.e. melee support, but fear late or mid game power spikes and scaling on the enemy team.

Starting E over Q helps your ADC CS and control the wave and can be better in sustained skirmishes. Q is better if you want to poke and deny the enemy ADC pressure to trade or CS.

E max is reliable for trades and scales the best with levels.

W max second helps you roam and is good in general.

Q max last because it has the highest AP ratio, but if you want to shut down an ADC in lane its also very viable to max first or second instead because it increases the blind duration, it just comes down to preference or the matchup.

Flash has always been the number 1 meta summoner spell since the beginning of LoL.

Ignite helps you pressure the lane and secure kills, but Heal, Exhaust, or even Teleport are all fine.

Heal if your ADC didn't take it, or two heals if you are versus insane poke, and it helps you apply Ardent.

Exhaust if they have more than one assassin.

Teleport can be useful late game as a cheesy filthy bush camping split pusher, or to roam and gank lanes and drop a few mushrooms and TP back to your ADC mid game, and useful in pressuring the lane in early game. Sometimes you can fake a TP from a bush onto another bush ward across the map and cancel it at the last second making the enemy think you're invisible where you TP'd, but really you stay and mind game ambush someone else.