First thing first
Ok I am currently S5 (my account is Servie2 on EUW) and I have only 75K on thresh but there are two reasons :

First: I don't like to play ranked
Second: I played thresh on a lot of account in total I would have around 300K on him

You guys can trust me on this guide

Please comment this guide with whys I did good and bad and like it if it's good
Enjoy

I tried 2 summoners spells. Exhaust and ignite. Exhaust is the safest option and guarantee a safe early and a great utility in team fights in late game. Ignite gives a HUGE first kill of the lane potential. If you are playing as a duo lane thresh + an adc with strong early damage like lucian, graves of draven and you want the first kill and you are not scared of their duo lane than go ignite

If playing against Katarina, Zed, Yasuo, Malzahar, Cassiopea and riven then I tecommend to go exhaust and try to use it on them during team fights.

Taking Flay (E) on lvl 1 is a must option however if you are invading do not take any abilities unless you see an enemy and than take Q and insta hook him.

Your Q take a few seconds before landing that's why you can predict a flash (Madlife Hook)

To predict a flash you need to be on the side of the enemy that's the easiest way. Just land your Q as if the enemy had already flashed and there you go.
Don't forget to time the flash if someone flash

Max Q for CDR and because all thresh player should do that
W lvl 3 to bring an easy gank or to escape and for the shield

I take mobi boots so I don't need to explain why I don't take the masteries that gives move speed in river

If you want a safer masteries page than go 0/12/18 with bond of stone
I take this page because I go with ignite and because it's better on thresh
If you go ignite and this page well than push lane early and all in lvl 2

I can't really tell you anything about matchups because it all depends on how you are playing as thresh and how good you are
To be fast squishy champion are easy but don't let them poke you and tanki matchups are easy early but hard in late
HARD MARCHUPS: Ok I will tell you 3 matchups always hard for thresh
1: Braum: He is very annoying with his shield so try to all in lvl 2 that's very important to be able to win the all in lvl 2 against a lane with Braum because when he is lvl 3 it's a lot harder to kill the ADC. Try to dodge his Q
2: Leona: She has a lot of CC and thresh is weak if his ADC gets CC. She has strong early damage so if you wanna all in early you really need a VERY STRONG EARLY ADC
3: Alistar: When you are hooking you are not moving so he can push you under tower and he is very tanki play aggro early and after wait for ganks care to his combo

HELP: You can outplay an Alistar easily:
If he Q your ADC, throw a lantern in prediction if he uses his W onto your ADC to push him to his ADC. Your ADC will just have to take the lantern (WORTH IT). If Alistar flashed in to do this that's really worth, time the flash of Alistar and ask for a free gank