I'm one of the many Agora refugees, trying to figure out which marketplace is best. Right now, I've opened accounts on Nucleus, Alphabay, and Abraxis. I've read a few security concerns surrounding Alphabay and Neucleus, although Alphabay has great selection. Abraxis seems solid (?), but has crap selection for Canadians.
Where's the best place to go?
I like Abraxas for the UI, alphabay's is terrible (along with the search). As far as selection, you're limited to the bigger markets: alphabay and nucleus I think. There is a thread here somewhere which sorts them by number of listings.
There is also the time factor to consider. All of the vendors and buyers are still in the process of migrating, so all of the sites are going to be growing for the next week or two, until agora finally pulls the plug.