Totally agree with DWibbie; mages are indeed a nice class to play 'cause of the CC (croud control) hability provided by AoE and the burst DPS of flame/fireball
As for secondary class, many things will influence your choice, for example:
1) Gear: since M wear cloth, if you use priest as a secondary, you can use the same pieces up to a certain level when having specific stats is important (lets say over lvl 60)
2) your play style: if you like to play alone: go for a priest secondary 'cause you can then self heal. If you want maximum DPS, then probably have a different secondary (not sure the most appropriate... S will be ranged too and boost speed (so it seems logic), R are the current "OP" class (there has to be one :p), ...
3) What secondary toon you want. With the triple class system, you can now effectively have two different toon in one. For example, my mage is a M/P but when I play my priest, I'm a P/K (not P / M). So chose a secondary you will also want to play for real (hence, level it properly to assist first class)
4) Don't forget, there is no "good" choice (there are bad choices tho). The important is to have a class you enjoy playing and will keep you playing and having fun!
Hope this helped