Very tough decision.  I would go with the type of stories that you enjoy reading.  Thus, Hawkman if you prefer the superheroes, Sgt. Rock/Our Army at War if you like war stories, or Viking Prince if your into adventure stories.  They were all originally published when Kubert was at his height.  The Tarzan books were later.  Kubert was still doing fantastic stuff but I thought that his mid-fifties to mid-sixties material was the best.