Gyuri, Nicole, Seungyeon, Sunghee. I don't listen to KARA4 days that much but Sunghee's voice is really easy to pick out. Gyul has a really unique voice, and I can always pick Nicole's voice out. :3 And I really like SY's voice and... yeah.
I need to hear Jing and Hara sing more, though. I can't distinguish theirs.