I use the BD ATC XP and ATC XP Guide. I also use the Petzl Reverso. I like them all. I use these different devices depending on different conditions (ie diameter of ropes, rapping on single rope, icy rope, etc..., do I need the extra features when bringing up one or two seconds, do I need the extra friction for whatever reason, could I use the auto lock feature in case something goes wrong). IMO, all these devices rappel smoothly. They are common, so most climbers you go out with will be familiar with them - that's a bonus. BTW, always backkup your rappel with a prussik loop. Just my $0.02.
For belay-rappel, My favorite is the atc guide. If I'm just rapping for the fun of it, I prefer a figure 8 because I can rap faster and more smoothly. I do have to be careful with the fig 8 loading the gate tough. I generally carry both to the crag; the fig 8 is a "just in case" belay device.
my fav? I rigged a hole in my #5 camalot (the previous edition) as a rap device - I only played with it on the ground, but it felt like it would work
a big hex! just watch out for roughed up edges from placing it, and be mindful of the rope rubbing on the sling if its nylon or spectra, a cabled hex would be best if you actually plan on this.
In Extreme Alpinism, Mark Twight said he used a #5 tricam as a belay/rappel device on a Himalayan epic....
the biner brake always works really smooth
as for my day to day favorite, the atc-guide for sure as the ultimate do everything device.