I have to agree with bisquitodoom, Saga of the Swamp Thing 30 is the best cover from the Moore run (I may have my own selfish reasons for that vote but there it is)!
I do not intend to hijack the "Purchase of the Year" topic, but with the recent dust up caused by the surfacing of the Swamp Thing 37 cover, I did a little basic internet digging into the covers for the Alan Moore Swamp Thing run and it looks like out of the roughly 45 potential covers (issues 20-64 and the Annual 2), only 11 are officially accounted for out there. Between CAF, Heritage, and CLink, I counted the following:
24, 28, 29 (which is - I think - an unpublished version), 37, 38, 40, 43, 47, 51, 56, and 57
So where are the others?