I think mine has to be, when I split up with an ex he had an amazing filleting knife that he left at my house, I packed all the rest of his crap up, but there was no way I was giving back the knife!!!
The other thing that was pretty petty. My ex husband was verbally and physically abusive to me for 9 long years. I never argued back (scared to) so I got my revenge in another way. He was a die hard Hammers fan and he had a vintage looking WHUFC polo shirt. He loved that shirt more than he loved anything (only cost about £30 so not much) and he was a larger man. Anyway, I was doing the washing and I 'accidentally' put it in the tumble dryer, yep, it shrunk, turned it into a very slight crop top, had to chuckle to myself when he put it on and I wasn't even sorry. He was so mean to me I needed to release some 'harmless non physical' petty onto him. It worked - he was really pissed off about it, I got about 5 mins of satisfaction. It was risky that I could have got a beating for it, but I think he was just too upset!!