rogue is the fastest leveller in the game but expensive, warrior second fastest and cheap, mage third and cheap. Ranger and druid both suck.
Oi, answers about Warrior and Druid aren't accepted, and you aren't supposed to mention Rangers
Rogues are definitely the fastest levelers, provided you are willing and able to use plenty of plat items for survivability.
Mages aren't that bad for soloing, they have some really good sustain skills that mean they don't need as much plat when leveling - at the cost of some damage, which means slower leveling. Mages are also under the unfortunate curse of not having any premium consumables that increase their damage output rate, unlike physical classes which can use hastes.
If you are looking ahead to endgame, you will probably have an easier time gearing a mage due to less of them making it all the way to endgame on the majority of servers...most endgame mages are alts, which depending on server may have less gearing priority than a mage main. On the other hand, it does get a little trickier for mages to level once they hit lategame and start grinding through the tower.
I personally think a mage is more fun to play, but that's just me. They are also quite a bit cheaper, especially at endgame. Raid fights usually only require an energy lix, whereas rogues usually need hastes, attacks, food, and more restos/idols (especially at prot

).