What is an unpopular gamemode that you think is overlooked and deserves more players?

If I had to list the best modes, I would definitely put Murder and Prop-Hunt on that list, which were definitely once the foundation of all Garry's Mod servers, just like TTT.
I could also recommend a rather old mode, which is called The Hidden. I admit, I don't remember exactly what it was all about, because I personally never played it, but I watched a lot of videos on this mode, and it looked pretty fun. I'm sure that if you do some searching, you can easily find its description.
In addition, Trash Compactor and SledBuild are also worth mentioning, unfortunetly both of which are already forgotten, but very entertaining modes nonetheless. Both Trash Compactor and SledBuild are best played with friends imo