I would actively vote against the liberal candidate for playing the "terror" card. His entire opening speech was dedicated to that, even though Londoners have probably shown themselves to be the populace who care the least about the "issue".
Boris Johnson is entertaining, but not exactly a professional. His points are vague, and mostly badly-thought-out copies of Ken's policies with funny bolt-ons.
Thus Ken Livingstone will get my vote. However much time he spends with his foot in his mouth, he has always impressed me by being honest, professional and capable. He has concisely and unambiguously defended himself in plain language in every debate, and I find this refreshing, especially in the current politics of vagueness and secrecy.
I'm rather afraid Boris will win on the sympathy vote, a part of which got Ken in last time. I hope not.