A 1960's era Colt Police Positive .38 revolver. After leaving law enforcement, I signed up for another weapons class aimed at civilians (was gonna use it for a security license, not being able to get a clear answer from the state then as to whether or not my LE training records, which were more than sufficient, would be accepted.) Just before the class, my own service-sized revolver was stolen, so I arrived at the class with only my Charter Arms Undercover .38 snub. The instructor wanted me to take the class with a service-type weapon, so he let me shoot it with his. Very nice and well-kept, with a smooth action and trigger. Shot quite well with it, too.
I also shot a rented Sig 1911 recently, too. It had definitely had some work done to it to tune it up, and it was nice to shoot. However, I can't let it claim the title of "most enjoyable" gun I've shot simply because I'm left-handed, and the gun was not, requiring too many adjustments in technique.