As much as I like Altiverb, the choice is glaringly obvious, I’d say: SPAT. Every mixing hour of the day, every day of the week.
In short: SPAT is endlessly more versatile than anything else and also sounds better — and I mean: the sonic integrity of SPAT’s output — than just about everything else. Its only major short-coming is the fact that, in most DAW’s, it’s quite difficult to make SPAT process several sources simultaneously (due to some routing inflexibilities in the combination DAW-SPAT).
But for single source processing, nothing even begins to comes close offering all that is possible with SPAT.
SPAT may look a complex, sophisticated beast at first, with a seemingly infinite number of enigmatic parameters, yes, but for our purposes — placing and positioning virtual instruments in a mix — you actualy only need to know about a surprisingly small handful of these parameters, no more anyway than the number of parameters you’re expected to know about when working with any other reverb/spatializer.
(A good understanding of) seven or so parameters gets you more than started, and then add, say, another ten or fifteen if you want to be fully equal to any spatializing challenge that a mock-up might ever present you with.
Also keep in mind that spatializing — good-sounding, mix- and music-aware spatializing, I mean — is a fairly complex affair to begin with, so a certain degree of commitment and a willingness-to-learn on the part of the user is always going to be requirement. And that’s not just the case with SPAT. Good use of Altiverb, or MIR, or whatever, can also only happen after quite a bit of studying the software and a thorough understanding of everything needs to be considered if you want to be able to spatialize virtual instruments convincingly.)
I’ve done a few SPAT-videos, going over all its important parameters and showing a few different techniques, using source sounds from Sample Modeling, VSL and LASS.
Here’s https://thesoundboard.net/resources/SB_SPAT_QuickGlance.mp4 (<b>one of those videos</b>), a "quick glance" type-of-thing which uses an anechoic flute recording to explain some of the SPAT-basics with. (The opening screen indicates where the other videos can be found.)
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