Different servers have different needs.
On Mistmoore.
Troubadors/Dirge
Coercer/Illusionist
Fury/Warden
Defiler/Mystic
Templar/Warden
Assassin/Ranger
Swashbuckler/Brigand
Conjuror/Necromancer
Warlock/Wizard
Ive listed them in order from what I can remember. Troubadors and in higher need then Dirges. But it also depends on the needs of a guild. My guild has 2 or 3 full time Drige's, but we dont have a single troubador. Other guilds might have troubadors but no dirge.
Also, youll find that the harder to solo classes are more difficult to level, and more rare at higher levels. The are also in higher needs at higher levels.
Tank classes are a little more difficult. While groups need tanks for instance runs and what not, guilds really only need two tanks. Anything more is almost a waste. Some encounters need 3 tanks, but really only MMIS and maybe EH need 3 tanks from what Ive seen.
If you want to paly a needed tank, be a berserker. They can do good DPS, can tank, and have gutroar. Something needed for some encounters.
If you want to be a healer, it all depends. Scouts, stick to either a rogue, or a bard. mage, try to stick to coercer/illusionist because everyone loves power regen.
just a little FYI.