Don't forget LO, photos that have a lot of chatter (comments) seem to float to the top along with the most downloaded. Their manual sorting system is awkward and my impression is that new uploads get lost. I think that LO will become a real player in the future so I hope they work on their sort order.
My own preference is DT's sorting system, quick and straight forward pull-down method. Relevancy, Most downloads, newest, oldest, and you can sort ascending and descensing.
When I purchase photos, if it is a popular category, I normally just sort by downloads descending, and a quick look at the first couple pages of newest.
Istock has the largest library, but I really loath searching for photos there. Sometimes their search system reacts like an overeducated snob.
I do like SS's "what's hot right now" approach. And it worked extremely well for me in the beginning when I had just a page of photos. I'm not uploading any of the old crap there and s-l-o-w-ly progressing, now that I'm up to 65 photos, I find that my popular ones cooling off. I think I was generating sales because all of the similars were right there on one page. Now they are spread around by popularity - falling on 3 pages. (Or, maybe the buyers felt for "the new kid with the small port" and grabbed 5 or 6 at a time.) I'm happy with my sales there though, for 65 pix I think I'm doing quite well. If we could just get those numbers at the 50% commission sites now!