Waifu, obviously- As a mage, I think we complement each other's abilities really well. I'm ashamed to admit I've disregarded the others so much, but I only took Hirge and Adaon as companions for their quests. Spent pretty much the entire length of the game with Grissenda. A quarter of the bag of holding is dedicated to all her weapons alone (gave Adaon the Shadow Blade as a token...after defeating the Void Lord with Grissenda)
Solaire would be my second choice, mostly for the Heal ability.
Sorry, Add-On. I like he's laid back personality and all, but he has the attitude of a 14 year old who just found out what the concept of an Anti hero is and thinks it's the coolest thing ever.
A suggestion, if I may - Long ago, I had the idea of a comic depicting the akwardness of the situation when you sleep in a bed, say, in the Tower of Tremadan, with a companion. Did Griss watch me sleep for 12 hours? Did she sleep on the floor? Did we...
https://m.youtube.com/watch?v=b_n_NgGWpR4
Due to my laughable drawing skills, I could never put that idea into action. If you like it, you can use it.