Almost all of their AI are challenging, and interesting in their own way.
Tagilla is fun for the slasher movie simulator.
Killa is fun, and can be either a breeze or a straight up nightmare one man army. I almost shit myself the first time he laid down suppressive fire on the corner I was peeking him from, then suddenly rounded the corner, power slid, and wasted my ass. Never saw that coming.
Shturman can be a breeze if youre the first to see him, but if he sees you first, thats a rough day.
Sanitar and his boys feel like a real life bad scifi movie, and can clap you from distance while making your bullets feel like nerf darts.
Gluhar and the gym bros are just straight up thugs. I mainly play solo, and I think I've only ever beat him and the boys 1 time. They're just so good at swarming, flanking, and grenading you with precision. They're almost comparable to players!
The only one I really find lack luster is Reshala. Him and the Super Mario Bros rejects are idiots and essentially throw themselves in the way of your bullets. But Customs is the "beginner" map so it makes sense that he and the boys aren't too cracked.
It just makes you really appreciate how, as someone with way too many hours in this game, I can still occasionally sweat going up against a boss, when most other games it gets stupid easy at one point.
Props to BSG