Denva is the most underrated tank of all the mercenaries! Now I haven't used him (or any other) mercenary on end-game content, as I mostly just solo/molo. However, when grouped with my Templar, Denva kicks some major butt! He also does well with my Fury too. I do not see him used very often, and he is not better than the SK, but as far as I can tell, is just as good.
Combos I have used so far:
SK/Inquisitor merc = Good, but old Stamper seems to think he is DPS.
Templar/Beserker merc = awesome combo!
SK/Defiler merc = awesome combo!
Defiler/SK merc = awesome combo!
Warden/SK merc = awesome combo
Beserker/Templar merc = Templar merc starts a bit slow, but becomes an awesome combo after level 40 or so.
SK/Warlock merc = fast killing, but Erudite has an annoying habit of (rarely) running about 200 meters away and aggroing every mob in the zone, then quickly dying!