Eder is the only one that's never left my party. I was planning on keeping all the original companions with me the whole time and just rotating the others in and out of that last slot but I ended up liking them more than I thought I would. I never take the sidekicks off the boat but Xoti, Serafen, and Aloth are probably getting the most party time after Eder though Maia is catching up.
I'm playing on classic and function and performance aren't a part of my decision making, just picking personalities I like. Honestly, I've never even looked at any stats other than my own.