You're putting yourself in a tough spot since the only real way you can find out which tank is "best" for you is to actually level them and try it out. But to answer you questions imo:
1. Dark Knight. Only needs one Dark Arts Power Slash combo to hold hate for an entire fight. In static/over-geared dps situations, you may require 2 but that's really it. For AoE, WAR since you can spam back to back Steel Cyclone/Decimate and if you know your rotations you can fit 3 into a berserk rotation which is great for burst or mob burn situations.
2. Physical goes to PLD and magical goes to DRK. WAR does both pretty well but doesn't specialize like the other two. Notable moves for PLD include Shelltron, Bulwark, Sentinel and Hallowed Ground. DRK has Dark Mind and Delirium.
3. WAR. End of discussion. It's possible to crit over 5k easily with Fell Cleave with just Maim and Storm's Eye up in Deliverance. It can go higher if other debuffs are applied and the situation is just right. My personal best was 7.4 k in Ravana EX but that was with the i180 weapon. Decimate does exactly what its name implies in trash mob situations like A2. Times that by 3 with Berserk and you may have to change your pants afterwards.
4. This one is up for debate atm. With DRK still being fine-tuned I feel like they all bring something to the table in different party situations. So no comment right now but imo I feel like in terms of utility PLD and WAR are tops. PLD has Clemency and Divine Veil and Stoneskin if you got CNJ at 34. WAR has Storm's Path and the ability to burst dps to help meet dps checks in fights, which is a must for most progression parties. DRK has Delirium but if you have a monk in our party then its useless.
As far as endgame tanking, I've seen mostly PLD/WAR but that's only because they are the most easiest to use. However I have seen PLD/DRK and WAR/DRK occasionally so it really up to the raid as a whole. If you are planning to do end-game then you might as well lvl all 3 when you can as this is the only melee class that you can get one set to gear 3 jobs proficiently.