A military press as it relates to their bodyweight. The closer to one's bodyweight they can get, the more impressive. Once they get past that stage and start measuring it in how many times their bodyweight I am extremely impressed.
I am impressed with Brad Gillingham's bench, given that he has long arms -- I am less impressed with short-armed guys who bounce BPs off their belly. Similarly, I have a decent military press, and even long arms, but if you look closely my upper arm is actually pretty darned short. No one escapes the general law of leverages, you can always improve, but the elite is always a genetic elite first and foremost.
Out of those two, a big and raw MP (drug free of course)...
Lifting heavy overhead is more difficult than lifting heavy laying down.
How I look at things is that if you need additional equipment (belt, wraps, suit, etc) to lift a weight than your body isn't ready for that weight. So you need to train up your weak links before going to that heavier weight. If you have a crain that's strong enough to lift a certain weight but it has a hook that can't handle the weight than that crain shouldn't be used to lift that weight. You'd need to replace the hook with a stronger one. Since you can't just replace your arms or core or whatever for a stronger one, you need to strengthen the weak link(s) before using the heavier weight.
I guess using help devices in competition and while training specifically for competition is ok. Personally if I was going to compete though I'd try to find raw competitions.
...its how much weight that comes off the deck that impresses me, not the lift that allowed it to happen. Plus, the deadlift is a real lift. The weight isn't just moving an inch or something contrived like if a guy raised five tons in the calf raise or shrugged a car or something.
A big military press. You need a lot more balance and technique to do a good military press. The bench press lets the bench do a lot of your supporting work for you. It's also, for some people, kind of a short movement.
I agree with MikeTheBear that one of the moves that has always impressed me the most is the clean and jerk, and that one better defines my idea of impressive upper body strength, which is I guess what you're going for, or you would have asked about things like squats and deadlifts. The ability to get a huge weight off the ground and then pressed overhead, to me, is about as macho as it gets, and the skill and balance factors are very high.