Thresh is considered one of the best supports in the game with amazing outplay potential. He is one of my favorite supports and functions as a tank and as a disabler. With multiple forms of crowd control, he is very useful both in lane and outside.

PROS:
-Incredible crowd control
-Potential to outplay
-Infinite capabilities and one of the few champions with infinite scaling
-Insane peel

CONS:
-Easily countered by champions with long duration crowd control effects
-Hard to master

Although Thresh is generally built as almost a full tank, I like to take most of the points in the utility page. This allows you to keep up with your relatively gold-hungry build while still being a late-game sponge. It also forgives you if you miss a hook with the mana regen and allowing you to chase more easily.

One of Thresh's best strengths is his build versatility. He can fill many roles that may be missing from your team comp. I generally like to build Thresh as a hard tank in order to act as the initiator for my team. Sunfire Cape and Spirit Visage are a must for a tank build and Talisman allows for an easier time initiating fights. Frozen Heart is a good first buy if you feel that you need to have shorter cooldowns, but if not then it is better to go with Sunfire.

I know many of you may be wondering why I max Q rather than E, and that's because the Q cooldown is significantly decreased and it does a lot of damage early game. This allows you to have more peel as your Q is your main form of CC.

Thresh has one of the stronger lane phases for a support. I tend to go aggressive as soon as we hit level 3, but if your ADC's pick doesn't support that, then it is better not to force it. Also, he has a hard time against champions such as Morgana, Lulu, or Leona. Really the rest is just up to situations and judgement. If you have the option to go in for a hook and you aren't sure, it is a lot safer just to not go in.

Late game is where Thresh truly shines as a play-maker. The better you get on him, the easier it will become to make such plays. You always want to be in the front line. In a fight, it is fine to hook the tank, as long as your team is there, you can Q into them and ult E. At that point the fight should start and you will be a target. It is easier to try to "support" your carry in the fight rather than becoming a full on damage sponge, although the latter is better to do if your are skilled and have gold. If you are not ahead or not confident in your ability, then after you initiate, get back and peel for your carries. With Thresh's AOE E and his many skills to lock down enemies, it should be easy to get any high damage champion off of your carry long enough for him/her to kite or run away.

In summary, Thresh is a very versatile champion with a kit that fits his role. He can be the deciding factor in games and has high carry potential for a support. He functions as a hard tank/peeler that starts fights and ends them with his AOE crowd control to completely lock down the enemy team. He has high early damage and near end game, can 1v2 carries by tanking damage and spitting it back out.