That is tough...I guess I would go SA Hawkman volume 1 if you are big into the super folks. Classic, classic work.

Otherwise, I might say go for Enemy Ace archives volume 1 just for the design/artwork alone (some complained that the stories were a bit repetitive)...

Of course the DH Tarzan by Kubert is also superb but that is technically Bronze Age I suppose.

Also of note is his own TOR creation. There were three HCs put out by DC (the covers have a "film" on them that seems to peel over time), and a lot of this work originated in the 50s. Later on there was a more recent TOR slim HC put out by DC.

Of course Rock is also a good choice, too. Tarzan and Tor are pretty much all Kubert throughout though...

So my priority (for you) might be:

SA Hawkman
DH Tarzan
TOR
Sgt. Rock