I haven't tried many combinations for long periods of time (just exploring), but I love Cole, Cassandra and Sera/Solas. I usually switch Cole for Vivienne in dragon fights though.
With Cole/Cass/Solas, there's some lovely banters there. Cole/Solas friendship is pretty obvious, but I was surprised how they both develop some kind of mutual respect/friendship thing with Cass. You'd think at first glance that a Seeker and general big-shot in the Chantry wouldn't get along with them, but no. It's really cute. Besides, in combat Cassandra can stand against pretty much any threat, Cole's does lots of damage, and Solas can use protective barriers and whatnot. Plus he has interesting insights on various things. Switching Solas for Sera loses the historical/magical insight but gains silly comments and humour. Her banter with Cass is alternately cute and funny. She's pretty horrible to Cole so I don't have them together for too long, despite loving them both.
On my next playthroughs, I'm eager to try Dorian/Bull/Vivienne, Dorian/Bull/Sera, and Varric/Cole/Cassandra. Plus the all-girl team. It depends on my character's class, race and romance for the best balance.