What classes are in high demand?

[Pyrion]

Mage is not over populated, there are not many mages around. But its a low utility class and not so much in demand.

You need to specify if you want to be in group demand or raid demand, that may change your preferences. While wizards suck in normal xp groups (AE groups is a different thing), they are in some demand for raids. So if raiding is your focus, a wizard might be a good choice, while for normal groups it absolutely sucks. Sustained DPS is really low.

There is always the holy trinity in groups. Tank/healer/dps. Those who do best at these tasks usually have an easier time finding a group. Enchanter/bard are also wanted because they speed up things for everyone, but there are quite a lot of them there.