I own Lexicon MPX, IK's CSR, Eventide' StereoRoom, Waves Renaissance Reverb and TrueVerb and Valhalla.
My most used is/are... by and large, the ones that come free with my DAW (Space Designer in particular). Same when I was working with Sonar - Sonitus were my go-to plug-ins - and I especially liked the reverb.
Each of the other ones mentioned above can be useful. If I had to pick one, I'd probably go with Valhalla. That being said, I don't use reverb half as much as I used to - I've never bothered to try any of those fancy reverbs that people are buying these days. The only one I'm quasi curious about is Waves' Abbey Road Plate.