I love pretty much every BioWare companion in every BioWare game I've played (minus BG, because I stopped early on when I couldn't save Xan without restarting, and just skipped to BG2). BG2, DA:O, DA:O:A and DA2.
Aerie, Velanna and Merrill are my favourites, but I also love:
Imoen; Minsc (and Boo!); Nalia; Jaheira; Yoshimo; Viconia; Anomen; Korgan; Keldorn; Mazzy; Valygar; Cernd; Haer'Dalis; Edwin; Sarevok; Morrigan; Zevran; Alistair; Wynne; Sten; Loghain; Dog (DA:O&2); Oghren; Shale; Nathaniel; Sigrun; Justice; Anders (DA:O&2); Mhairi; Ariane; Finn; Sketch; Aveline; Fenris; Isabela; Varric; Bethany; Carver; Sebastian; and even Tallis!
I completely missed Jan in BG2, but his quotes make him really funny.
Not Tug, Silas, Jerrik, Brogan, Runic Golem and Snug the Bronto. I think Leliana's Song and Golems of Amgarrak sucked, as interesting as the thaig was. The companions weren't interesting except Sketch - I liked his side quest in DA2.
BioWare really makes some of the best characters, especially playable ones. On par with Fallout followers. I honestly don't understand how anyone could hate ANY BioWare companion. On that note, I also love NPCs like Isolde. The only char I didn't really like was Rendon Howe.