For me, it was Solas. It's not that I outright hate the guy -- with his deadpan eccentric personality, I just thought he was the least interesting out of all the companions. His connection with the Fade and his views thereof were fairly interesting, but it wasn't enough to really engross me like other companions did.
Also, from a gameplay standpoint, as much as I wanted it to be my favorite mage specialization going in, I just couldn't get into Rift Mage like I did the other two.
Once I started collecting more companions and therefore had more options for party builds, I benched Solas and there he stayed for the vast majority of the rest of the game.
Ironically enough, though, by the end of the game, next to Iron Bull (who I romanced, and who wasn't too thrilled with me for a while after I had made the decision to dissolve the Circles), it was Solas who liked me the most. Seriously, the guy jumped for joy at every decision I made. I don't think there was a single time in the game where I got disapproval from him.
Other frequent bench warmers included Cassandra, and, sadly enough, Sera.
Solas.
Much like you i found him quite interesting whit all the fade talk and explonation of the spirits. Its trully a good info dump on two legs. Sadly somehow he become boring rather quick and whit far more interesting mage ( Vivienne ) he become a character i used on few occasions for quests and character recruting.
Iron Bull.
I really liked him. I found him awesome in every way possible. Problem is i play as a Qunari 2h fighter and i just cant stand to remove Cassandra who is by far one of my favorite characters. So Iron Bull just stays in Skyhold doing nothing. Really sad couse i do like that guy a lot.
Sera.
At first she was my "only" rouge in the party. Funy, crazy, chaotic and if she was LI for streight male i would have been whit her in a heartbeat. But then Cole came. The spirit of compasion. He is the only character who can shed a lot of light on every companion you have. Each of his banter dialogs are lore. He is also have very very fun mini game in skyhold if you listen to the banter of the random npc's there and then confront him about what they have said. So Sera just fallen behind by huge maring.
Blackwall.
Like him but ... despite reminding me of my gray warden i just cant get interested in him. Cool looking character but just not interesting.
Dorian.
Remind me Anders way too much - and no not the cool and funy Anders from Awakaning. The annoying Anders from DA:2
Varric.
He is compeating whit Sera and Cole - he flatout losses in every possible way.