"Best speakers" depends a lot on your own opinion, your ears - and the depth of your pockets, I suppose.
Staying with B&W, but on a (slightly) lower scale, there's this set
Scroll down a bit on that page. Looks like the company wants you to believe that's a picture of the speakers connected to an iMac.
Having connected speakers to an iMac, I can say with no reservations that the best speakers for it are simply whatever speakers sound best to your ears. The fact that you're connecting them to an iMac is pretty much a non-issue.