This question comes up every so often. I think i've used them all and I've always come back to RARExpander. Particularly as I use alot of password protected archives and this is the only one which hasn't given me any grief with this. But in general too, this is the best one I've come across. For as long as I've known it it hasn't been updated, though. But having said that, it doesn't really need to be imo.
I've run into other programs not being able to decompress multiple rar files selected at once (ie; outputting .mkv files), especially when a password is needed.
I'm in on using the 2.x release of The Unarchiver. It can handle .rar files much better now.
And taking the rar.icns from this app http://www.macupdate.com/info.php/id/31609/rawr! (which is also an unpacker) and put it inside The Unarchiver, then your rar files will look like they are integrated with the system because the rar files will look like your zip files if you use the Archive utility in OS X.