As there really isn't an answer to the first part --- I know a lot of people throw around a bunch of numbers but none of it is really going to make an awful lot of sense, I'm going to jump to the more important part:
On a related note, if everyone in the world had the sense to use the volume knob on their playback devices (i.e. without the loudness war), what levels do you aim for?
I'll step in this one because it was an early thing when I was first dragged kicking and screaming out of my comfort zone and into the mastering chair (long story that I won't get in to at the moment).
When that happened, the intense study began. And all of it was done at fixed output levels (I still feel that people have no idea how important it is to have a properly calibrated chain from start to finish, but that's for another thread). Low volume, mid volume, high volume, low end, midrange, top end, apparent compression at what point, apparent limiting at what point, how are the effects and space 'moving' (was there a compressor on the reverb buss or is it 'breathing' from the mastering stage), etc., etc., yada, yada.
One thing stood out almost immediately and hasn't changed since (and this was 1993-1994-ish). A ~15dB crest factor (lets broaden that to "14-to-16dB") tends to be where pop/rock/metal/rap (non-acoustic, non-classical, modern and contemporary recordings for lack of a better term) is happy. And to that end,
I have yet to hear *any* recording with less than a 15dB crest that didn't sound better *with* a 15dB crest.
Not that I'm trying to say "universal" -- but it was close. If recordings came in with a 18-24dB crest, you could almost bet that while decreasing it to around ~15dB would add perceived "power" and "energy" and "groove" and all those descriptors that people love attached to their music. Open, dynamic, powerful, unconstrained, fast, etc.
Past that point -- I mean RIGHT past in most cases -- Start pushing the crest down to 13 or 12dB and you've killed it. Squashed, constrained, slow, unexciting, irritating, etc. You've not only removed the listener's
reason to crank it up, you've removed the
reward for it.
Is it a number to shoot for? No. I don't go by numbers and meters (except during the all-important calibration mentioned earlier). But it was a number that came up so frequently, nearly universally, that it was worth spending the last half-hour trying to type a reply to a forum post.
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And note that although it's easy enough to move the "crest factor" numbers into "dB(FS)RMS" numbers, I'm sticking with the former because that's what really applies. If a project calls for -24dB(FS)RMS levels, such as high-res or motion picture projects, that same crest is where the music is probably going to sit.
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