In terms of what party would be most effective in battle (from a lore perspective), I'd go with this:
Mages
Offensive
Morrigan - she's the child of a god after all and a highly skilled mage capable of an enormous range of abilities from shapeshifting to blood magic.
Solas - an actual god is even better, and one that can twist the power of the fade.
Defensive
Wynne - she's highly powerful and supported by a spirit of faith (and not in a crazy quasi-abomination way).
Vivienne - as leader of the Circles, she has to be one of the most powerful mages and healers around.
Warriors
2 Handed
Sten - he's a hardened Qunari warrior whose strength is demonstrated by his position as Arishok.
Fenris - for his invaluable lyrium abilities.
S/S
Justice - for his remarkable spirit abilities that put him above most other warriors.
Cassandra - the strongest of the seekers who are one of the top military organisations with devastating powers.
Rogues
Daggers
Zevran - he has outsmarted and outfought hordes of antivan crows with his brilliant mind and fighting abilities.
Cole - for his invaluable spirit abilities.
Ranged
Leliana - she has years of experience as a bard, soldier and hand of the Divine which makes her one of the better archers.
Varric - his eagle eyes and top-of-the-range technology make him one of the deadliest archers in Thedas.