I haven't collected patches (or even owned a denim jacket) for years now. My leather's taken a battering so I'm even going to need a new one of those soon too.
My collection was quite impressive back in the day as it were. I kept the arms on it for more patchery and if the memory still works properly, I had:
Large back patch of Anthrax - Spreading the disease
Celtic Frost - The Emperor's Return
Death - Leprosy
Kreator - Coma of Souls & demon head
Anthrax - State of Euphoria/Among the Living/Logo/NOT Man
Slayer - Large logo on the back across the top/small logo/South of Heaven
Destruction - Mad Butcher
Dio - logo
WASP - logo
Helloween - Pumpkin + logo
Iron Maiden - 2 x Eddie (Aces High & Run to the Hills)
Megadeth - Peace Sells
Metallica - Ride/Puppets
There were a handful more but I just can't remember what they were. I remember you could only see about 2 square inches (aside from the collar) of denim underneath the patches though. Twas quite impressive. Shame it met with an ex-girlfriend related stealing incident.
