For people who say that Firefox is a memory hog, I say that Chrome is actually worse, especially after you get over six or seven open tabs. Large numbers of tabs are also a problem for Opera, according to what I've read.
To me, Opera is flat-out missing functionality that should come from having a working addon architecture and a developer community to go along with it, especially now that Chrome has had a decent number of Firefox addons ported to it. Opera still has weird keyboard shortcuts and partially implemented chunks of what are great ideas when carried to their logical conclusion, such as its pain in the ass content blocker that doesn't offer subscription-based updates.
As far as speed, the crown seems to be basically passed to whichever browser has released most recently and depends quite a bit on what's being included in the benchmark.
With some thought I guess Opera is probably better than Safari as well. I forgot about Safari. And at least they don't expect people to pay for it any more.