I must say kataang seems most likely. It's been developed since episode 1. And although I'm a shipper of Kataang, I think realistically.
Katara seems to have been devolping stronger feelings for Aang at the beginning of season 3. Possibly even before that. I mean, I don't kiss my bestfriends on the cheek. Hugs, yes, kisses no. And Katara having "Only motherly love" for Aang is a poor arguement. Katara simply is motherly. I'm the same, even with my boyfriends. Bsides Mother's don't kiss their "sons" back. O.o (DOBS)
I could go on for a loooooong time, but I'm sure I've made a valid point. Okay, I'm leaving for right now. (Turns and says dramatically "I'll be back")