Having been playing Thresh since season 3, I've tried him out in plenty of different areas, even including the jungle, and managed to make it work. But I can tell you from personal experience that he is strongest by far when played as a support for a few reasons.
First off, his damage scalings aren't that great. They're certainly nothing to write home about, although enough stacks on his passive though getting souls can still start making a difference. The Box is his only high AP ratio ability, and it's janky to hit with, and has plenty of base damage of it's own to where you don't really have to worry about getting more damage on it. Building him as an AP Tank doesn't work very well because his AP based damage is very meh. Aside from his ult, his other damaging abilities have low base damages and average or below average ratios. Their worth is in their utility more so than their damage, and so building AP on him is a pretty solid waste. Nashor's Tooth with an on hit build can help to alleviate some of the repercussions of an emphasis on AP by adding to the on hit damage from his Flay, but you have to invest so heavily into it to get benefit that you're losing out on tank stats that he needs to be effective in favor of AP that he's only getting half as much benefit from as most.
The AD ratio on his flail is a bit of a trap in all honesty. It only reaches maximum bonus damage if you don't hit someone with him for a long period of time, several seconds. So building attack speed items on him ensures that you're pretty much never going to be getting the full use of his Flay since you won't have time to charge up sufficient bonus damage between attacks aside from the number of souls you have. If you're building him as a fighter or an ADC, you aren't doing it because of the damage on his Flay. It's only ever going to be relevant if you spend several seconds between hits, and when going into a fight you're only going to get it on the first attack you land.
You dealt an extra 300 damage on a build that's already critting for more than that on regular auto attacks once at the beginning of a fight. To anyone other than a carry it's laughable. Damage oriented builds are about sustained DPS, and his passive damage boost is a small bonus when he's going into a fight. It doesn't function the same way when he's dealing constant damage. You're giving up the full use of the ability that made you consider running him as an AD champion in the first place. Running him as an ADC, especially with a Ruunan's, is worth nothing more than dealing his bonus damage from a fully charged Flay passive to three enemies at once, plus proccing three bursts at the same time from a fully charged Shiv. A build like that isn't effective because it's good on him particularly, it's effective because it's a standard ADC build. If someone lets you get to the point where you have all of those items finished, you're going to look like you're doing a lot of damage because you're sitting on several hundred AD, probably with some lifesteal, with enough AS to keep the chains coming. It's not because HE is strong, it's because his ITEMS are strong.
I love Thresh. He's a great concept, his design is tons of fun, and he has room for a lot of clutch plays. But because he brings so much utility in his kit, not damage, it's best to keep him where he belongs: alongside someone who he can protect with his W, set up for kills with his Q, disengage with his ult, and keep in place with his E. You'll carry a lot more games by learning how to land a hook through a minion wave and knock people into more than one Box wall than you will by focusing on your own damage.
I'd suggest building him similar to how you'd expect someone to build a Leona. I honestly don't build support items outside of the initial Talisman/FoTM, because that's about all the supporting effects he needs. He's already got a shield, and giving him an active movement speed boost only augments his ability to peel and disengage for carries or to close the gap and lock down a target to secure a kill. After I'm done with that I just go straight tank and CDR. Being able to send out more Qs, knock people up with more Es, shield and save with more Ws, and drop more ults goes a LOT further than trying to be a damage dealer. Sitting in the middle of the enemy team controlling who goes where and when is what makes him a terror when he's built as a tanky support, and they're forced to group up when he's around to keep their high priority players from being hooked and deleted by Thresh's team.
All in all, the best advice I can give you is to have fun. If you enjoy a champion, play them where you want how you want. But understand that whether we like it or not, champions are designed to fill certain niches. Just because you can make them work in some ways doesn't mean that it's necessarily a good idea to play them like that all the time. More often than not, playing a champion outside of their standard role well usually comes as much from your team helping you be successful as it is from you making the right choices. On top of that, playing a champion outside of their role too much makes Riot have to respond by nerfing them if they're becoming oppressive, which weakens their abilities in their main roles.
Have fun, but use caution. Understand what a champion is and what they're made for, where their true strengths lie, and learn how to play them well enough to carry from their main role before you try to break the meta.
and I couldn't think of a last item, I tried out 
and 
and looking at ADC Thresh builds, it seems 
and 
are decent on him, too.
basically adc thresh because lord have mercy on the soul who gets trinity passive, E empowered, crit auto attack.
BoRK's active range is the same as Thresh's autos. if you get a crit depending on your soul count, you first hit will chunk people for around 1800 mixed damage before mitigation. the mallet and furor combo makes it hard to run from him if you don't have mobility. i've noticed that we fail hardest against poke/heal bot lanes.
