I think mine would be Rift Mage couldn't play it in my first run as I couldn't get the 3 venatori tomes I needed to unlock it in the end so went with Necromancer was still good fun though. I think after using Stonefist and Firestorm in the first 2 games I would have preferred Rift mage though as I loved those spells as they made me feel super powerful bringing fire down on everyone's heads
The thing with Rift Mage is that it is Thedas' version of an Adept. Singularity is Pull of the Abyss, Warp is Stonefist, Slam is Veilstrike and Biotic Effects are Weakening.
It is awesome to watch the boom-boom and combos but after a while you realize that the Rift Mage is a one trick pony of sorts.
I myself prefer versatility. For example in DAO, I played as a Shapeshifter. I played on PC where there was a mod called Combat Tweaks which fixed all the issues you had in the game so Shapeshifting was awesome. In DA2, I played as a Spirit Healer who used offensive spells meaning I would mostly be offensive and turn on Spirit Healing only when necessary.
This versatility is gone in the mage and mage specializations in DAI. Knight Enchanter, especially with the recent changes turned it into an even more combat mage oriented class as opposed to a hybrid class. Rift Mage is an elemental adept.
Necromancer could theoretically be the versatile specialization since it comes with crowd control, damage, damage over time, pets and time control but alas, that specialization is buggy as hell. I heard Necromancer is awesome in Multiplayer.