1. The best pure tank is Blackwall, the best 2H is Cassandra (as a support warrior for buffs horn of valor and templar) the Bull isnt that good cause the AI sucks at using the Reaver skills.
2. Sera is the best stand alone rogue damage wise, Varric is better in a party with high critical chance value, Cole is good for bosses because of mark of death.
3.Dorian is the best pure offence mage but requires a lot of micromanaging to shine, Vivienne is the best support mage because of the barrier bonuses in the Knight Enchanter spec, Solas is the best stand alone mage for 2 reasons : weaken and Pull of the Abyss.
Depending on what spec you will choose and what difficulty you playing on the general party is different on normal and lower anything works really.
Necromancer and Rift mage have very good synergy with each other as it helps set up walking bombs combos with pull of the abyss so if you are rift mage go with Dorian as your mage and vise versa.
If you are a Knight enchanter it doesnt matter anything goes. Generally I found the best group for a mage is 2 mages 1 rogue archer 1 warrior tank.
That is all my opinion and based on my experience in the game this far, hope it helps good luck.