I would say Tough Mudder. I've done three so far and they were a lot of fun. I'm signing up for another in April. So far I was amazed at how different they all were. What's nice is that while cardio is important, you don't need to be able to run the entire 10-12 miles at one time. The obstacles help break that up.
I haven't done the Warrior Dash or Spartan race, but I would imagine they're more for fun and not anything you need to train for. The last Tough Mudder I did had a sign three miles in that said "If this were the Warrior Dash, you'd be done by now."
I'm doing the Super Spartan in Temecula at the end of the month. Its 8 miles plus obstacles, so not sure about the 'don't have to train' comment above. Granted I am not doing any special training for this. Just my usual tri and strength training. I do agree though, the Tough Mudder seems to steps it up a notch.
I'd have to agree that the Tough Mudder is the best, though I haven't done all the different brands yet. By far the most fun and challenging event I'd ever done. Did my first solo as a personal challenge, but would only do it in a group in the future, just way more fun that way. Planning to do at least one in 2012. I also liked the Men's Health Urbanathalon, but it is more focused on the run than the obstacles.
I've done all three, including various Mud Runs and similar races. Of them all I'd say Tough Mudder is the best and most difficult.
Warrior Dashes and the like are great starter races to test yourself out - find your weaknesses and strengths. They are designed for maximum public participation, which is great. They are hard as you want to make them - meaning that the effort you put in is what you will get out.
Tough Mudders on the other hand, will seriously kick your ass regardless.
Tough Mudder GA on 11 Feb? Hope to see you all there.