I probably don't have some of the best stuff like the Martyr Wraps I keep reading about unlocked, but...
Tri-Sword: Does wonders any time I have to run a Halfling or Gnome.
Midas Gloves: General "I don't know what's out there" prep; I can always take this and extra apothecaries (or shop scroll) and be largely prepared for most situations. Converts well when all non-boss enemies are dead.
Platemail: If I'm going in with a physical resist class like Monk, or know I'm going to be gaining resists (Binlor worship, the 5% magic resist charm imps, et cetera.) I had a L10 Monk on the troll bridge level with 75% magic resist, 68% physical, and Platemail. Suffice it to say that was one of my easiest runs.
Crystal Ball: Breaks casters wide open, even with the gold cost (I've seen from old screenshots it used to not have that.) I never want to see this on the same map as Midas Gloves. Good thing I can't prep both.
Vampiric Sword: General good melee lovin'. Bonus points if I know I'm going to be going in with Dracul.
Whurrgarbal: The best answer I know of that largely melee classes have for dealing with Death Protection. Rarely prepped, but generally a lifesaver when I do. I imagine this is what Martyr Wraps would replace.
Something probably needs to replace Platemail too, I'm just not sure what.