Flash = Flash to make flash plays and also to escape dodgy situations.
Exhaust= For assassins and fed players, (you will need this in solo q alot, lol)

Relic Shield= If they have a very poke heavy lane, and you might need to heal up your adc, try using this item.
Ancient Coin= If you have a normal lane phase where you think you can make some plays, try this item.

I prefer both "Eye" items over the "Face of the Mountain" and "Talismon" because you get the sightstone build into it, this allows you to have another space open for more items =D.

Frozen Heart= I like to get this item really early on as it results in high CDR, this allows me to make more plays, the tankiness is also quite helpful against AD champs. Attackspeed reducstion also helps =D.

Mobility Boots= I usually go for these boots because I like to roam and try to be as helpful as I can for my team, this also allows me to do the first back cheese; this is when you go to midlane after your first back and try and get a firstblood or a free kill on the enemy midlaner.

Banshees= I've decided that I like banshees on Thresh for a number of reasons, the main one is the passive, allowing me to not take and dmg or effects on the next ability hitting me, when you're the engager this can be really helpful as the enemy team will usually use CC to try and stop this. Banshees completely counters this. While you're charging your Death Sentence you're also immobile, leaving you prone to getting caught with CC.

Flay Max First= The reason I max Flay first is because I think its Thresh's strongest ability, and allows him to get the most out of his kit, early game the Flay damage from the passive allows strong poking, while it can also be used aggresively and also passively. The Flay is also a displacement meaning you can flay people out of important abilities, such as; Leona's E. You can use this inlane to your advantage as the enemy leona will never be able to touch your ADC aslong as you're between her and your AD.