If you like Green Lantern, then Atom is a must. I like to buy Showcases as it gives me a chance to try out material I haven't necessarily read before and which I wouldn't buy in Archives as some of them are really quite poor and aren't keepers. Hawkman is worth a go for the art by Kubert and Anderson. The stories got a little weaker over time.

The Jonah Hex and Enemy Ace Showcases are both superb non-superhero material.