Mage, most likely. I've played around with a different character for every class build (and the occasional mixed build), but my very first character ended up a mage because of the sheer utility it provides. It's as solid as any class in the early game, remains solid throughout the entire game, and picks up even more steam near the end. In particular, I've found it peaks at the very beginning and end - the former with the Gem Staves, Waterbolt (if you got lucky), and especially Space Gun, and the latter with the Spectre and Nebula armour sets.
Nebula armour in particular deserves some attention. I personally consider it the single best endgame armour set with incredible durability (life nebula), incredible freedom of attack (mana nebula), and incredible damage output (the already powerful Nebula Pillar/Moon Lord spells plus damage nebula). This isn't even getting into the magic weapons themselves like Space Gun, Demon Scythe, Nettle Burst, Razorblade Typhoon, Last Prism... the list goes on.
A very close second would be melee, which is itself quickly followed by summoner - but both only in the very late stages of the game, and both mainly because of the phenomenal Valhalla Knight armour pieces. When it comes down to it, magic will always be my weapon of choice.