There are so many nice ones, it's quite hard to choose. I remember feeling very triumphant when I bought my second Red-Box Ballantine paperback set. Those (and several others) had Tolkien's own paintings on the cover: Hobbiton, The Forest, and Barad-Dur. But it was the Heraldry on the red box that (as a young teenager) hypnotized me for hours on end.
The much newer boxed paperback set with the illustrations of Frodo at the Ford, Sam & Frodo at the Emyn Muil, and Gandalf approaching Minas Tirith are another favorite. The Hobbit has Gandalf approaching Bag End which has been the Del Rey cover ever since.
Diamond, the Red Book is lovely! Have you seen the Green Book?
Saraphim, go ahead and get a new set-- just take good care of your old one and never throw it away (like I did, fool that I am. It was mildewed, more's the pity.) So don't store yours in a damp basement like I did...