I voted for What is Love. While the other two singles are fine, I like the instrumental for WIL better. It's got a bit of a swing beat to it that I like and the dance is funky and amuses me every time I watch it and It's great live.
Kimi no Kawari is my 2nd pick. It has the bones to be an awesome concert song. But I really don't like the instrumental at all. It's all the pauses and then the opening of the song which reappears again at the end. I wish it was something else.
Egao, while it has some great moments in it. I love it I really do. But I hate the live version. It's not a very good concert song. Maybe they'll grow into performing it, but I've been very underwhelmed at every performance I've seen of it so far. I also don't care much for the rap singing parts.