The 'best' is always relative, but here's a general review of some of the big ones from my experience.
DirectAdmin - Very good, stable, low resource use, and easy to get familiar with. It is also relatively inexpensive, unlimited lifetime licenses are only $299 per server. Most other panels require ongoing expenses, but this one does not. On top of usability, one of the primary benefits of this software is existence of
http://www.site-helper.com which lists step by step how-to's on nearly every function of the software.
cPanel - Very thorough, and a lot of people have had experience with it. It is a little more costly, but most people that have the extra $600/yr don't complain. That price is for a dedicated server, so with a virtual hosting account with cPanel the cost isn't that high...but your host will still have to pay that cost and pass it on to the you in the end.
Plesk - Clean, versatile, and a lot of expandability. The cost ranges depending on your use, and there are ongoing yearly expenses, but the software looks good and is pretty thorough. It also offers a lot of add-ons, including game servers and their functional application pack. The new Plesk Expand is one of the nicest options, allowing multiple Plesk servers to be joined into an integrated hosting platform.
From a host's standpoint, I have have had limited experience with Ensim and HostFlow as well. But both seem to carry a lot of overhead expenses, are processor intensive, and lack the wide knowledge bases of the other big names.
The best thing is to find a control panel that you and your host is comfortable with, and go from there. They all have pro's and con's depending on your needs and resources.
Hope this helps!
Chad