I can't pick just ONE, so here's my ever-growing list:
1. People with poor hygiene.
2. People with staring problems.
3. People who leave their shit all over the place (towels on machines that they aren't using; plates on machines that they're finished using because they are too damn lazy to put them back where they belong).
4. Dumbbells that aren't put back where they belong because d-bags are either too lazy to put them back on the rack, or simply can't count.
5. People who interrupt me for stupid small talk when I'm in the middle of a set.
6. Resolutioners who take up all the parking spaces. When will they quit already??
7. People who will sit on a machine and text for 20 minutes, instead of do their damn sets.
8. I belong to a sports club and if I go too early in the evening or on weekends, kids are running around doing group or swimming classes and every so often, one will run over close to the free weights while their fat ass parents are too busy chatting the other parents to realize.
9. The fat ass old women who hang out in the locker room all night, then shower and go home. I don't even know what they do... swim or hang out in the sauna, I think. Why bother paying almost $60/month for a gym membership though, if that's all you're going to do?
10. Women who want to chat in the locker rooms. There's one girl I see every so often who's serious about lifting and who's pretty cool, but aside from that, I'm not there to make friends!
Umm... I'm sure there's more. People piss me off in the gym ALL the time, lol.