Gotta give a huge nod here to ML Sound Lab's "Best IR In The World". I've used it a bit in live applications thus far, and I've found that it's a fairly universal IR for a lot of different amp models and tone styles. Bit of a claim, for sure, but it's free and sounds rad. The cab they submitted for the Gift of Tone series ("Mikko's Cab") is very similar, and worth snagging as well.
http://bit.ly/MLSoundLab
Bit of a tip from my experience, too: I highly recommend finding a single IR that that gets you as close as possible to the tone you want. Mixing IRs can be a real worm hole, and the phase differences that occur when you add another IR are often difficult to judge outside a full mix context. Find a few IRs you like and strap 'em cross the channels of your Cab block, and then use a Looper block with some chugs/open chords/single note/dynamic playing to give you a basis of comparison between them. It can be really interesting how certain IRs flatter different playing styles. These days I tend to find myself gravitating towards IR's that do all of them well, and excel at none.
