Gotta hook em' all
Example build (against heavy ad)
Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Why movement speed?
As Thresh you have to move fast to catch enemies to flay and hook them for your team or get to allies that are in trouble, with this build it shouldn't be a problem. I personally don't build anything else becouse I find this build perfect! You could also take ghost instead of flash to make sure the enemies have no way out. With movement speed you can ward much quicker and overall have better control of the map.