Undeadman by Norihiro Yagi is his first published work and it has everything that could be found in his later work, Angel Densetsu. It's a very Yagi-esque work. I know that's a vague thing to say but I can't put it into words that well.
Undeadman is about a boy named Shinanai, who was turned into an immortal or an "undeadman." He escapes from the lab where he was being experimented on and goes to his crush. He then returns to the lab with her to become human once again. Hilarity ensues.
Undeadman's art is rough but clear and good nevertheless. There aren't too many characters in Undeadman, but the ones present are done well.
I quite enjoyed Undeadman. I read his other two works, Angel Densetsu and Claymore, and was interested in reading his debut work, and I wasn't disappointed. If you liked AD, then you'll surely love this.