I actually really love all the companions, so kudos to Larian and the VAs for bringing them to life <3
If I had to put them in a ranking, it would be:

1. Minthara
2. Astarion
3. Karlach

I find it funny that Minthara is ranking so highly on my list despite having the least amount of content, but I think her character is just that good. Her and Astarion are both very complex characters with interesting backstories and their VAs also left the biggest impression on me, just very memorable voices overall. Also Karlach is just mwah, chef's kiss. Her response to trauma is something I can deeply relate with. She deserves all the love she's been getting and I hope they decide to expand on her down the line. For 4, I can't decide between Gale or Shadowheart. I love their designs, their respective character arcs, and both have the most beautiful romance scenes in the game:
out-of-body experience vs beach/in water scene.


Minthara bug list, updated for Hotfix #22.