Thresh is that one support that's always been a good support. His CC and shields are perfect for an support. He's one of my favourite supports and always will be. You can easily carry yourself out of lower Elo's if you main thresh.

is a must have these days, it can be used offensive to land a hook or defensive to get away from the fight.

is really nice for thresh if your team could use some extra CC.

is nice if your midlaner doesn't have Ignite or/and you want to go aggressive on bot.

You go most of the time relic shield and upgrade that, but sometimes you can go acnient coin. Ancient coin is really nice if you need an extra engage on the team. you can upgrade relic shield to eye of the equinox or face of the mountain, FotM is better if you want to protect your ally but Eye can be nice if you want an extra items slot to be tankier (especially nice if you're the only tank in the team).

against Full AD/AP Comps is pretty basic, just buy a lot of armor/magic resist. If the enemy team is balanced you'll always have a different build. the notes on the items explain it the most.

Maxing Q after lvl 6 really makes it easier for you in the teamfights, being able to pull as many people as possible and having your engage on low CD is always nice.

0/12/18 is right now the way to go for Thresh support, getting extra gold and extra tankiness is always nice as a Thresh. Bond of stone will always be on because you're always close to somebody in your team (bot in lane or roaming to different lanes). I've taken bandit here because that's really nice if you think you're gonna stay in lane long to make use of it. if your team is based around teamfights make sure you take dangerous game because the heal will save you a lot.

PROS

Super tanky in the mid and late game once you get a few items.

Very aggressive lane bully.

Can make or break teamfights by catching people off guard and getting 'hooks' onto enemies that are out of position.

Brings a lot of cc to team fights and the protection and utility provided by lantern is overpowered.

Is a ranged support with high damage auto attacks which means even if you miss your death sentence you can still bully your lane by harassing them from a distance or lanterning your jungle in for a gank.

Probably the most fun of the strongest supports currently.

CONS

Skillshot reliant; there are easier champs that do what thresh does just as well at the moment (annie)

Takes a lot of practice and hundreds of games to master simply because of how many skillshots he has he has a very steep learning curve.

No sustain in lane (all or nothing champion)