While he never worked much on the superhero comics I loved so much as a kid, his Tarzan made quite an impression on me. (click to see full size)
Where I first encountered his work was with the 100 Page Super Spectaculars (which I tried in vain to collect, but my newsstand wasn't as interested in them as I was.) I did manage to get my hands on this one at the newsstand.
As a kid, Kubert's style struck me as extremely realistic - not in a photo realism way, but in ability to capture motion. Even simple pages with very little action looked realer.
When I was in college, and heard about the Joe Kubert school for comic artists, it made total sense to me. Who else would be better suited for such a role?
He was definitely the master of the form.