What are the best addons for a TTT Server ?

It depends if you're more of a vanilla TTT or a creative TTT.
I recommend getting the basic perks for detectives and traitors, etc..

Pointshop is nice if you can utilize it properly for TTT.

Best not to overfill the server with junk stuff that really shouldn't be there so keep that strongly in mind.
I usually find a bulk amount and and mentally run it through a series of tests and thoughts before i even add to my workshop.