You don't choose the loot, the loot chooses you!
Or, more seriously ...
I try for an organic loot method (based on the situation and where the loot is found, rather than wish lists or player usability).
if they're raiding the wizard lab, items that would have been useful to the wizard would be there like a stuff that boosts rituals, and ritual components, etc.
If possible now and again I do spin something towards an item better suited for one PC or another who might be low on magic items (i.e. the loot up until that point had no interest to him/her). But, beyond that, if they don't use it, they disenchant it and use it to upgrade existing stuff.
Of course, all this goes with the caveat that I, as the DM, know my players are behind the expected curve and I actually do monster-side adjustments to compensate (i.e. if I know all the PCs have +2 weapons instead of the expected +3, i lower enemy defenses by 1; if the pcs all have +1 armor/cloaks, when they are expected to have +3 by that level, then i lower the monster attacks by -2. and so on). so i keep notice of if the PCs are getting hit too often or not hitting enemies enough, etc.
If I was not willing to do DM-side adjustments, I'd probably consider doing some sort of wishlist type system and every couple of levels ask for a fresh/updated one.. not that all wishes would be fullfilled, but it would give me a guideline on the types of things to pepper in.