Delta is definitely up there. Check out my review of it
[ame="http://www.youtube.com/watch?v=Far53N4y2kE"]DX Delta Driver Review - YouTube[/ame]
The drivers from W are also up there. Also the Decadriver.
The main reason these drivers are up there is because of the sheer amount of playability in them. Delta is very interactive, and the activation can be done by saying or doing just about anything, which ups the playability by a lot. The Decadriver, you could pretty much print off and make any kind of card you want and so long as you have the generic rider barcode on it, you can role play however you like. And the W drivers, if you're any decent with electronics, are the easiest to really customize.
Also up on my list of fun belts are the IXA Knuckle and the CS Kuuga belt, as these are very interactive as well. Here's the breakdown, my personal top ten belts from best to not as awesome
Delta Driver
Double Driver
Decadriver
Lost Driver
Accel Driver (below Lost Driver because I don't like the aesthetics)
CS Kuuga
IXA Knuckle
OOO Driver
Chalice Rouzer