I think space designer is one of the best reverbs out there for the simple fact that you can download loads of amazing impulse responses for it, Trillian space is also great and has a great selection of impulse responses that also can be used in space designer. I have also heard good things about valhalla, lexicon and Aether ones but havn't tried them. I generally just use a combination of space designer, Rverb (which i think has really nice sounding plates) and sometimes True verb