While not underrated in the larger sense, E.C. Segar's Popeye is the greatest comic ever and every bit as funny and modern now as it must have been 65 years ago. My vote goes to the English translations that are finally hitting the market from Humanoids Publishing. I really hope these guys finally find their audience. Still the consistently most fun, most surprising biff, bam, boom, for your buck. Erik Larsen does a great job.
All comics spring from that well.
The best comic you've never read. Tommy Yune is a wonderful talent. Anything Brian Michael Bendis is working on. Torso, Jinx , take your pick. If you love good super-hero comics with a twist, you really should be reading this.