All of the above, but especially smelly guys. Okay, I know it is a gym but you do not need to wear the same workout outfit all week and keep it in your gym bag in your car in between workouts. Ugh, the stench could bring you to your knees. Don't they smell it when they put on the clothes again?
That, and naked people strutting around in the locker room. I don't want to see it! Towel up. Fortunately most of the women in my gym don't do that but we used to have one who gave herself a complete breast exam every morning in the locker room mirror. It was gross and very uncomfortable.
And my personal pet peeve is the women who get on the stairmaster for an hour, hunched over and grabbing the handrails, butt sticking out and taking teeny steps on their tippytoes. It hurts to look at it. And I don't understand it.