What is your favorite mod?

There's a Sonic fan game I like to play called Sonic Robo Blast 2, built on an extremely heavily modified OG Doom engine with a pretty good modding community, and there's a level pack for it called Sol Sestancia that's just crazy fun to run through with the Neo Sonic character mod. Getting up to top speed to activate boost mode and trying your best not to slow down or stop so you don't lose it. So satisfying.