Well the valuable items are of course the lvl 10 arties, but also the saddle (ar9) can fetch around 4 mils +-. Also the standing swords and the flowers are desired. Oh and in my opinion the most decorating stealable is a white lamppost
Most times it´s luck or sheer patience to get one of the higher ones, but I know there´s some info on the spawn frequency on stratics somewhere.
My best steal was the blade of rightousness in doom 2, and it was pure luck I got it *grins proudly*
Edit: I found the spawn times. Here ya go:
http://uo.stratics.com/content/atlas/doom.php#stealing