Paladins win this race and nothing even remotely comes close. Paladins are probably the only tanks in the game that have a very natural flow to it. It isn't clunky, or mashing of keys. It simply flows and is very smooth.
Paladins are pretty decent when it comes to soloing though Death Knights do it better.
Monks are a tough one. They seem a bit more clunky when I tried mine. Often times I was waiting for Energy to regen. It took me a really long time to Master Death Knight tanking which I would consider hard managing two resources, but Monks are a step or two above that.
Paladins are probably the easiest and fail proof tank in the game. Its hard to screw up on a Paladin. They have so many passive mitigations compared to say a Warrior or a Death Knight. Even a Death Knight has to use their CDs in tune with Death Strike to get maximum benefit.
Warriors can be quite a pain since they lack in the healing department and are often clunky and require a ton of mashing of keys.