If you have nothing to start with a DK is good because you start at 55, so get into legit boosting territory faster.

Overall a prot pally I think is best, but that is what I use. But I can easily boost through every instance up to UK - I haven't tried higher. Though at that level my little guys are helping some.

Really any level 80 should be able to boot at least through SM and probably Strat. So if you have a toon at 80, use that instead of leveling something new. You could include a prot pally in the first group of toons to have one eventually use.