I use Mindscapes, BoEM I & III and parts of II (sorry Henry -- I can see where your players are coming from.

Not to mention the bard does nothing to correct what I percieve as the problems with the bard; get Path of Magic for that instead.), BoHM, and Requiem a fair bit. The way that the poll is arranged, I would have to say BoEM series, because collectively, I have probably used it the most. Individually, BoEM I has been around a long time and I have had the most chance to use the spells and classes from it, and BoEM soul magic and spells have seen a fair bit of use in my game. BoEM III I love the locations but haven't had a chance to use them; mostly I have just used the scrying rules (which are great.)
Mindscapes hasn't been around very long, but in any campaign in which there are psionic characters, it gets used at least every other session (i.e., whenever the party runs into a psionic opponent.) The mindscapes combat system works great and I was releived to see that it will work well with XPH.
Choasitech I haven't had a chance to use yet, but it will figure prominently in an upcoming campaign arc.
Requiem figured prominently in a recent campaign and if I ever get back to my sailors campaign, it will figure in a related campaign.
BoHM I've used here and there, but only occasionally. I would like to use more; there's some good stuff in there.
I probably should have voted mindscapes, as the slant of lumping three books together seems to be pretty telling for the poll.