My favorite Valonqar would be either:
Jaime: Obvious. Who would have a better reason to kill Cersei? She´s pretty much the only person in the world he trusts and now he finds out that she is betraying him like everyone else. Might be he needs to do it to make sure she doesn't kill Tommen, might be he does it in a rage or might be he kills her by not helping her.
Sandor Clegane: I do not know why nobody here has written this (or did someone?), but I think it's definetely a possibility that Sandor is the Valonqar. He has an elder brother and a good reason to kill him.
It would fit so well, if Cersei would choose "trail by combat" (against the faith) and name the monster, Qyburn's working on (
Gregor Clegane) her champion and the High Septon names the gravedigger from the Quiet Isle, who we all know is probably Sandor.
I have not read Dance yet, so we will see if this is total nonsense...
Kevan: Is a younger brother and he doesn't really like Cersei that much. So... why not? But I actually don't see him as an potential Kinslayer.
Tyrion: In my opinion it would be to obvious, because it is exactly what Cersei fears. But before Martin lets her live on even Tyrion would fit.
Loras: Not very likely, because wounded, but he will protect Margaery at any cost.
Stannis, the Stark boys, Danny aso. would be lame in my opinion. There is no personal connection to Cersei at all.
Sry for bad english (maybe?)