It's all very simple.
Warriors
At the high end, warriors are unquestionably the best tanks. With an aggro weapon and a proc weapon that both have decent ratios, pulling aggro from you is virtually impossible (except for that damn first cleric in the CH chain) and your tanking ability is second to none.
On the downside, you suck at soloing, and you won't even come close to shining unless you're at the VERY high end. Warriors are all potential that few will probably ever come to see the fruit of.
Shadowknights
For the part of the game most people will see, shadowknights are probably the best tanks. They can produce good DPS with Spears, melee and other spells, they can hold aggro very well as long as mana lasts with their aggro spells. They can tank sufficiently well, too. An average shadowknight is probably preferred over an average warrior in an exp group or on a raid, for now.
Additionally, they get lifetaps, FD and self-procs not to mention a pet, making them efficient both in soloing and grouping as they can act as both tanks and pullers and can, in theory, help the healer out at least a bit.
Paladins
Paladins are much like shadowknights up until the high end game, except they can't pull but instead heal very well. In the high end, SKs serve as DPS and offtanks and paladins serve as pad healers or group healers. As far as I understand, they solo reasonably well.
Rangers
Rangers are part tank, part DPS. In an exp group, they can tank or deal DPS, and in the high end game they're highly desired for their bow DPS (once that gets fixed, anyway). They're also extremely efficient soloers since they can bowkite.
Rogues
Rogues are all about DPS. That's all they do and that's all they can. DPS is alpha and omega for a rogue.
Yes, you could in theory tank a little if everyone else spontaneously goes AFK, but you're not gonna get any groups based on your tanking ability.
