My recommendation would be the same as for the Fourth World comics: the must have issues are the Jack Kirby ones.

I also have a personal fondness for the few Thor issues leading to (and including) issue #200; they don't feature the Eternals per se, as they don't get to do much, but they resolve a major plot point from their book. They also manage a remarkable feat of continuity weaving, successfully blending Wagner's tetralogy, Jack Kirby's Eternals, the purpose of that doomsday-heralding Odinsword, the purpose of that ultimate weapon the Destroyer, and the new gods story from around Thor #202-203.