Pretty much just about all toons are in demand again low level since everyone seems to be PLing there toons up. But Make whatever you are comfortable playing. With boon being put to live servers, Xping has moved to being soloable. You can xp on white and yellow mobs to gain a few levles then complete quests.
If your lucky enough to group be sure to send a friends invite to all. But Pretty much pickclaws in hpare best way to get lvl 20 then hope for a group to break that boring way to level. But picks are good for starting capital as well.
I would have to say healers are in demand low level but not so much high up. If your doing a DD go with monk cause they can own but A bow path ranger can rock as well.
Caster's Are a toss up Enchanters are good due to charms and pets. Mage's will almost always have a group at Trees once higher lvl and coth. wizzy's own with mass damage and ports. Notsure if nec's count as a caster but got good damage pets do rock and if built right decent heals. plus depending on path give power to group.
Alchs are awesome and in need but are tricky to get a good build.
So really all toons have there advantages. But it seems tanks and healers low end are always in demand.