LoL
I’d say my core is stronger now from doing all free-weight, unsupported lifts like T mentioned. Even doing something like chins is night and day from sitting down, locking your legs in place w/ some machine. If you work through heavy front squatting, overhead supports, etc., your core should be pretty damn solid. Well, that and using Suzan Sommer’s ab cruncher machine.
Re: overhead supports. I think they’re a fantastic core strengthener. Similar to OH squats but you can use more weight. You just lock it out in the OH position, walk out of the rack a bit, and statically hold using delts/traps/core. Also helps w/ confidence in putting up big OH presses.