Welllll...I say 'Kya' more than anything, or 'Kyuuuuu' or even 'Kweh?' when I'm confused.
But I like both mekyo and hyuu and - although I can actually whistle, albeit not brilliantly...haha...- I tend to go 'Hyuu' when confused or surprised.
Kyu for when I'm tired and bleh.
Kya (or Nya) for when I'm happy.
Hyuu and Kweh for confusion, and in hyuu's case, surprise as well.
Half the time I don't realise I'm making those noises until my friends go: ...Aw, you just said 'kweh/kya/kyu/hyuu' again!
XD;;