In patch 3.09 (current wow) DKs have 2-3k more base HP, quite a bit more armor (up to 3-4k), and higher avoidance (via blade barrier, which is up a LOT and provides 10% extra parry) than prot pallys and warriors. Its not a theory, its a fact from EJ, tankspot, etc that DKs consistently parse lowest damage taken on fights like Patchwerk or 3 drake Sartharion. They have much shorter cooldowns on 4 core abilities - Icebound Fortitude (20-40% damage reduction buff based on defense, 1 min c/d and up for 12 secs), AMZ (16k "anti magic" bubble, 2 min c/d), AMS (100% magic damage shield for 5 secs, 45 sec c/d), and Bone Shield (20% damage reduction, 1 min c/d). These are significantly, significantly better than Paladin 50% dmg reduction shield wall on (3, 5?) min cooldown and Holy Shield, or warrior Shield Wall.
Now of course patch 3.1 coming out soon changes a lot - the tank abilities of DKs are changed a bit, feral druids have had their HP reduced a bit, warriors got some new stuff, prot paladins had some stuff worked around. So far top guild parses for hard mode bosses suggest still DKs are the best tanks as far as survivability is concerned but the differences are smaller now, and its unlikely tank changes are going to happen before patch 3.2 now (six+ months away).
The biggest issue is that shield block is not a very good tank ability right now. Its great when you pull 50 low level mobs and watch in glee as they kill themselves attacking you, but it doesn't do much when you are fighting Hodir and he melees for 18k a swing followed by 40k magic damage - woot, I blocked 1800 out of a 18k swing, but damn that 40k breath killed me. Thats where DKs with a big arsenal of anti-magic damage tools really shine. The Blizzard designers have acknowledged that there are significant flaws with shield block, and I wouldn't be suprised to see it reworked in patch 3.2. But again, thats a long, long, long ways off.
The original question is whats the best tank for multiboxing, and unless you love paladins for some reason the answer still stands that 99% of the time Death Knights are the best tanks currently in 3.09 and most likely in patch 3.1, barring any last minute changes in the next few weeks.
This is from someone btw who has a 4/5 t7.5 prot pally who was my main for 2 years and MT'd every raid instance except Sunwell, I rerolled a DK 2 months ago and it is really a lot better tank. Just for example with my prot pally Utgarde Keep - Skadi is hit and miss, the adds can do significant damage in the first few seconds of the fight to my pally, whereas I can go a good 40 seconds without a heal on my DK (this is the farm 5 harpoons and do it all at once strat).