I may get laughed off of GS for this, but I'll play: two of my favorite verbs happen to be free or VERY affordable:
Apple's "Matrix Reverb" (included in OSX), and Magnus Jonsson's "Ambience" (donationware) -
Magnus' plugins
They both do things I've never heard anywhere else, and can sound absolutely stunning when tweaked. Matrix Reverb does a room simulation that I find unbeatable for adding realistic "air" around certain things (Leslie cabinets, drums & perc) as well as really good long verbs for pianos, vocals, etc.
Ambience is also capable of really good results. Both of these verbs have beat out the big guns for me on MANY occasions, and I've got 'em all. In fact, on several score projects these two have gotten me REALLY close to that elusive Abbey Road Studio One sound on orch tracks - when the budget doesn't allow for the real thing of course ;-) They're now both staples in my scoring template.
Space Designer can be great too, but like most convolution verbs, it's a matter of what you feed it...