Aw heck this just isn't fair, we can only pick ONE???
I've got just about every Mongoose release over the last 12 months, but if pushed I have to choose B5. It's my all-time favourite show (although it's being threatened by Alias), and after the disappointment that was The Babylon Project
seeing a good RPG with the amount of support Mongoose have given it and being d20 system to boot made it everything I could wish for.
The B5 core book was the second Mongoose product I ever bought, just after the Dredd RPG. As a longtime fan of GW's Dredd game I surprised myself by consigning the GW boxed set to the archives after picking up Mongoose's version.
DancingDryad.com was pretty much founded on a mix of Mongoose books and LotR Minis, with the occasional other product scattered in there, and to be honest thats still our two major product areas- although the Mongoose Minis are now threatening the GW stuff