The next best thing to a real room/hall is a Quantec unit. Its algorithm simulates the pressure differences in a room, instead of the usual approach of creating an approximation of wall reflections via delay lines. This results in a physically correct response to different stimuli and makes it possible to send an entire mix through the unit and still get a clear and convincing result.
I've got a an old QRS from the early 80s, which is not as powerfull as the latest Yardstick (though still more powerfull than the one from the 90s) but has more of a mojo to it. It's filled to the brink with cards stacked with chips. Sweeping sinewaves through it you can observe the maxima and minima shift between the channels (up to 4 for surround), dependent on the room size parameter.
It's one of those units that aren't very obvious unless you take it out of the mix. Of course, you can also do sublimely exaggerated reverberation, which gets you very close to the sound of classic records, IMO.
Sadly, the creator Wolf Buchleitner died too early a few years ago. An avid piano player, he had his epiphany regarding the resonances shouting into a piano and observing the subsequent 'reverberation'. Nobody has so far managed to create a convincing copy (or even an approximation) of the algorithm, which I believe requires very sophisticated fine tuning to work this well. There's a plugin that claims to incorporate the QRS algorithm, but to my ears it's a far cry from it.