I keep my default on My Newsmakers so that I can first look at what stories the people I'm following are clipping. I then usually also glance regularly at the My Networks tab so that I can discover fresh content.
My Networks is sorted by time of last activity across all of your networks. We then also feed in one Hot Right Now | Recent item per day per network.
You can zoom in on a particular network by hovering on the name of that network. I think what you're asking for a list of what is most popular today across all your networks? We could do that pretty easily.
I prefer My Networks, but mostly because I'm watching for people to add to My Newsmakers. When I build a relevant pool of Newsmakers, I'll probably switch.
@Jasongoldberg: Well, both a list of what's popular across all my nets *and* a list of what's new. As it is they seem grouped by network, so I rarely have the attention span to make it all the way back to World News =)