Well I am currently playing a Dagger rogue so don't really need another one of those. So spec other rogues to be archers. Spec warriors to be SS tanks, don't like 2H and 2H overlaps daggers too much.
Always one tank, which can be any warrior. The game seems to be built around there always being a tank.
AND
Two range character or one range plus Viv. At least one of which is a mage.
Possible to try two range plus Viv probably works ok.
I tend to go with story first. So if I think a character fits the story I tend to include them. Or the missions has some connection with their personal quests.
Blackwell for missions involving Gray Wardens or Dark Spawn.
Varric on missions involving Red Lyrim.
Solas for scrambling around accent tomb and archaeology stuff.
Vivienne missions that involve difficult combats.
Any other suggestions?