I don't know why but Kopaka has been one of my favorites for a while. I like the swordsman aspect to him, I really like the choice of making his sword more on his wrist. The spindly build of the character actually works well with the older figures design.
I always see Matoro and Lhikan and Jaller as my three favorites because they've all made the ultimate sacrifice at one point. Jaller sacrificed himself as a Matoran to save Takua from being killed by a Rahkshi (he was later brought back to life, but still), Toa Matoro gave his own life to awaken Mata Nui, and Turaga Lhikan saved Vakama from Makuta Teridax's Shadow Hand. Though if I had to choose one favorite, it'd be Matoro, for he saved the entire matoran universe by allowing Mata Nui to awaken.