My two best trophies are thus:
The Grey Messageboard that sits right in front of the porters lodge. Stole it, re-arrenged the letters slightly, returned it. Stealth-action at its best.
The ultimate one was the large hot food trolley from Van Milderts MCR dining all. Unfortunately we got 'caught in posession' and my room mate got banned for a year. We went back for a pint a week later and no-one said anything, mind.