I think it's Latoken. I'm not sure if it's popular or not but it's pretty easy to use and very secured at the same time. I recommend it to young ICO projects since there are a lot of active and big traders there.
To be honest, Latoken was not really a good exchange from my opinion, well they rank on TOP 50. But because of a lot project listing at there, the volume trading was really small. Until now i still doesn't use Latoken.