Stay away from anything that says Master on it and absolutely stay away from combination locks, both the dial and the wheel style. You want a lock that uses bal

Stay away from anything that says Master on it and absolutely stay away from combination locks, both the dial and the wheel style. You want a lock that uses ball bearings in the shackle, not the spring loaded latch type (you can easily tell which is used if you unlock the lock and inspect the inside of the shackle; if you see perfectly circular notches on the inside of the shackle then that lock uses ball bearings. If you see a straight edge in the cutout of the latch, you're dealing with a spring mechanism). The American Lock 5300 is a nice choice for this application (yes, it is made by Master but it has some redeeming qualities). It is shrouded so it cannot be easily shimmed and it comes with a 5 pin replaceable cylinder. The keyway on mine is relatively paracentric (many steep angles and warding that makes it harder to pick) which is nice. If you are super paranoid you can re-pin the core or swap it with a new one, preferably something with 6 pins. The best feature of this lock is that it looks more secure than its neighbors and it cannot be easily shimmed or opened with a kinetic attack. If I were popping gym locker locks I'd go kinetic on spring mechanism locks since they are the easiest to open quickly with no special tools. I wouldn't want to be caught with a rake or a bump key so those methods are probably not your biggest concern. You want the thief to look at your lock and think, shit, this one isn't worth the effort, move on the next. Like you said, you bought sub-par locks and they were simply forced open. A 5300 or similar isn't going to give it up that easily, which makes you less of a target.