First of all UFC fighters are not the best as people portray them to be. NHB/Mixed Martial Arts is very tough and the competitors are very tough. The majority of NHB now a days are just fighters instead of Martial Artists. You have strong big guys who know wrestling, brazilian jiu-jitsu, boxing, and muay thai. The majority of the UFC is just ground and pound and is sloppy. There is no art in that. I bet you if you were to see REAL Martial Art Masters such as Morihei Ueshiba, Steven Seagal. Mas Oyama, Bruce Lee, Hee Il Cho, Jigoro Kano, Chun Sik Kim, etc they would win because they are true martial ART masters. Remember martial arts is not just about the physical and fighting aspects but also is about the art, the best fighters/martial artists focus on the art aspect as well. You really have not seen real martial art masters in NHB only a few such as 1994 Olympic Judo Gold Medalist Hideko Yoshida, Rickson Gracie 7th degree gracie jiu-jitsu blackbelt, argueably the best nhb fighter and grappler in the world, the rest of the gracies, and a couple others.
The point is there are many more martial art masters who do not choose to compete in NHB but they are certainley better than these strong NHB fighters out there. Just imagine putting a REAL Martial Arts Master like Mas Oyama vs. UFC fighter Rico Rodriguez, who do you think would win?- Without a doubt Oyama.
So in my opinion yes Seagal could beat most of the UFC fighters out there, but remember Seagal is now older and does not focus on NHB fighting but of a more humble and effective art, Aikido.