Well, I'm terrible when it comes to tanks, and tend not to bring them along. Tend to play rogue, so I might keep only one more in the group Etc, Etc.
If ignoring such things, and focusing on companions as characters rather than fighters, I would probably go through the new ones and keep those in such a combination that they entertain. It would in the end depend on how the fighting system works, and which ones I'm best at using for it.
But just to pick based on assumptions from conceptart and such.
Sera, Dorian and Iron Bull/Solas.