Actually nowadays my most rewatched (or rather endured) tv shows are Teen Titans Go and Sponge Bob. Darn school closures!

Mythbusters<<<<<<<
X-files
The Stand
Samurai-X

Seinfeld
Futurama

I am more of a movie guy.

Black Adder and Battlestar Galactica are probably my two most rewatched series.

I don't really rewatch stuff, in fact I can't remember more than 1 show I have watched twice in its entirety.

That show is Code Geass. I plan to rewatch it in another 10 years or so, or if a friend wants to watch it with me.

The Simpsons even though I haven't seen all of the newer episodes I pretty much seen all the earlier episodes at leat five times or more.

Comedy series are most re-watchable, like Curb Your Enthusiasm, HIMYM, Seinfeld and The Office (US). I must admit, Seinfeld feels a bit dated today but I'm still a fan.

I've also watched Twin Peaks and Lonesome Dove many times. 1st & 3rd seasons of True Detective are also outstanding and re-watchable.