What? Arcane Warrior was by far the best solo class/spec in Origins and in 2 there's no real contender because it's quite possible with any class (if optimized, of course) because of runes - though it's way more difficult with a Mage than, for instance, an Archer or two-handed Warrior.
Arcane Warrior was the best class in Origins, yes, but a Dex-built rogue could solo just as easily on nightmare. And any mage that abused Gravity Ring or was a Spirit Healer was the easiest solo class in 2, but any rogue with cunning as the primary stat would survive just about every fight with as much ease.
In each game, the first ten levels were the hardest in regards to soloing for each class... I expect Inquisition to play similarly. The early game will be the hardest to get through soloing, but a well built rogue or mage would be the "easiest" classes.
I have every intention of trying to do a solo playthrough, but it certainly won't be the first few attempts. I'll need a few games to learn the strengths and weaknesses of each class, as well as how the combat system has changed, to pick a class I can play adequately enough to do it.