Bank checks aren't stealable. House deeds are stealable one hour after the deed is created. The best items to steal would probably be skill orbs, taming skill scrolls, master control diagrams and aspect cores/extracts.
To add items to what others have said. BOATS(!), Casino deco rares, T-maps, good ore maps, supremely instruments, spell hues, rare cloths, dyes, and phylacteries.
Rare exceptions things lesser are worth stealing. Like band-aids from someone macroing flamestrike at the edge of town and then placing a spell damage rune just outside town in range of them so they kill themselves. Always farms a good chortle.