For free there is SIR
http://www.knufinke.de/sir/sir1.phpWaves IR-1 has had good reviews
and you can usually get the IR-L for reasonable
(same reverb with less features)
and has a LOT (4.8 gb if you download all) of Impulses
@acoustics.net, which, if you have a Waves account, you can
download anyways.
PerfectSpace is good if you're willing to do the work to get it to sound good,
and if you have Xx you should already have it.
And here are some Donationware Impulses that have high ratings..
http://www.resoundsound.com/4-impulse-response-reverb-packs/