Mage. I was actually quite hooked onto summoner for a while, but then 1.4(.1) came along.
What is there really to say about mage that hasn't been said? You get amazing useful spells which you can cast from safe distances. Mana, while being a limitation, isn't a "finite" source like ranger ammo - rest for a few moments, and it will regenerate without the need for any potions (and of course, if you use potions it will replenish even better). And one of the many things I can definitely praise this class for is how weapon variety is handled - not only do almost all weapons feel different and unique (with maybe the exception of the first few gem staves, but those can be forgiven since simple weapons are needed too), but you also have the choice of which arsenal you want to use - whilst you can spam a single weapon for most fights, the game also tries to encourage the use of multiple weapons through utilities such as clinger staff and rainbow gun, and by making different weapons serve different purposes (for example, nebula arcanum absolutely dominates in arenas and invasions, but the nebula blaze is more reliable when high speed aerial battles are in action). Compare this to something like ranger, where daedalus memebow or onyx blaster can pretty much carry you until golem, or summoner where sanguine and blade staff nullify the existence of any other pre-plantera (and even a few post-plantera) summons.
I haven't really said much about melee... That is because I just generally dislike being up close and personal in combat.