If you’re worried about your chances of winning with new players, then you should just stick with Warrior.
Paladin has great party utility and support, but that means that they’re heavily dependent on the party to actually make use of that support. In the scenario you’re describing, even if you play it the best you possibly can, your team will almost certainly waste every opportunity you give them. Pld’s don’t have enough Dps to cover for weak Dps teammates. Their defensive utility is strong, but it’s not strong enough to cover for a weak healer. In the end, you have to rely on your team to carry their own weight. If they can’t do that, then you’re not carrying them anywhere.
Dark Knight, on the other hand, is a bit of a wild card. Their recent changes make me uncertain of their capabilities within the arena … but honestly I don’t see a bind being a big game changer. With a coordinated group, I could be very useful, but with a bunch of randoms? Not likely. Fresh players, especially, are usually all over the place, so any binds you apply are likely to be wasted or thrown out right off the bat. Carnal Chill is pretty handy for shutting down bursts, but it takes really good timing and awareness to use. It also won’t make up for a weak healer. They also have a strong burst, but if you’re looking for that you could just stick with War.
War’s don’t really suffer this drawback. They lack practically any healer support, and their party utility is only moderate, but they don’t rely on the party for pretty much anything. Weak heals? No problem. War can self-heal more efficiently than Pld. Weak Dps? No problem. War can burst for comparable numbers. Do you require any support from the team to do either of these things? No. Does your party utility require the team to be aware of it to work (like Pld stuns or Drk Binds)? No. War’s can operate without having to worry about their own teammates screwing them over. Pld’s and Drk’s cannot. War’s can also contribute to kills without being solely dependent on the Dps to have their heads on straight. Pld’s and Drk’s cannot. Between the three, War is the easiest tank to carry with by far, assuming you know how to work your burst.
In the end, if you’re looking for the easiest tank to carry with, then War is your answer. It doesn’t get easier than that.