Best mage specialization?

It depends on what you are trying to achieve.

If you want to optimize damage throughput, this Necromancer build is one of the best: fextralife.com/forums/t162540/build-pyro-necromancer/

If you want to mix melee with magic, a KE equipped with the Blade of Tidarion is a viable option.

If you want to control the battlefield and flatten fools, then blow them sky high, go for a Winter Rift Mage: fextralife.com/forums/t176497/winter-rift-mage-great-solo-build-maybe-one-of-the-best/

[...]   Against that final dragon in the Emprise du Lion, Sera and Dorian kept into trouble with the little dragons coming to its aid so in the end I just ignored trying to revive them and finished the battle with just me and Cassandra. It seemed appropriate really considering she was meant to be from a family a fabled dragon hunters.

[...]

Sadly, (for me now anyway), I'm beginning to agree with you.  The focus powers are frequently throw-aways, although the one for rift mage is pretty neat -- so long as your target doesn't move out of the impact area.  For rift mage, Stone fist seems to be the only worthwhile power.  Necromancer it is....next time.

deadlydwarf , there is no need to regret your decision to play Rift Mage. My canon run was a Rift Mage and it's still my favorite mage spec to play for fun factor. Sure, it's crap for damage compared to Necro and KE, but neither of those specs can flatten all the enemies and keep them on their backs for an entire battle. I breezed through Jaws of Hakkon, none of the Spies ever got close to my rift mage, because I'd hit them with Veilstrike as an opener, then put a Searing Glyph under them, and if that didn't kill them by the time they were back on their feet, a Stonefist would finish them off.

You just have to figure out how to make the most of the RM skills and configure the rest of the party to help. I usually ran with Stonefist and Veilstrike only, not Pull of the Abyss, because I wanted the other slots for three damage abilities: Searing Glyph, Energy Barrage, Winter's Ruin, and then two defensive abilities, Chilling Array and Barrier. Notice, no Shocking abilities, due to the Asleep vs. Weakened combo issue.

Since Veilstrike and Stonefist are going to keep a lot of enemies on their backs, you want party abilities that take advantage of that, like 2H warrior Mighty Blow and  Combat Roll/Coming Through or a rogue throwing an Antivan Fire grenade. A second mage with Searing Glyph (Dorian) or Flaming Array (Viv) doesn't hurt. An Archer with Explosive Arrow or Leaping Shot into the flattened mob is always helpful.

Don't lament the lost of firepower, just figure out how to utilize the RM abilities to their best effect and you'll have fun. I know I did.

For point #1, why take your mage and turn him/her into a squishy warrior?  In DAO, you had the middle option of being a spellcaster whose status as an arcane warrior allowed him/her to do melee combat in a pinch.  With only eight usable powers at a time, you have to focus one way or another in DAI; can't really be a "jack of all trades." 

The theory of rift mage sounds cool.  (Wasn't there something like that in DA2 that worked pretty well?)  Unfortunately, the only active power that works for an Inquisitor is Stonefist -- a good detonator.  Some people swear by Veilstrike, but I find it underwhelming and as to pull of the abyss, I find static cage in the Storm Tree more useful.

Next time, Necromancer.  The active powers are more effective and "mage-y."