Some recommendations:
- WTF Story
- Ethology: The lost chapters
- Terri's night out
- Rave Story 1 and 2 (the sequel is pretty okay)
- Schism
- Jenka's Nightmare
- Lost Land (If you want something more simple and straight forward in terms of a CS sequel mod, I see it as the lesser of the 2 compared to Jenka's Nightmare)
- Carnage Clash
Those are some my personal picks, there are a few challenge mods but they come far and few between and vary in quality. Some like Carnage Clash are great, and a few others can boil down to being stupid-hard, i.e. Story Off the Rails and Ballos' Return (Ballos' Return is a poor attempt of a challenge mod and Story Off the Rails is a little on the bullshit side despite being well made).
I heard it's super good and possibly the best mod in the world
there's going to be an update soon...