I'm the oddball. I voted for push ups. Actually, I find all three exercises equally challenging with simplefit. Just challenging in different ways.
With pull ups I can do a bunch and am limited only by my strength. I stop only because my body simply won't do any more. But no soreness or exhaustion. Its like its easy for a while and then suddenly impossible.
With push ups I first feel general fatigue, but can keep pressing on. Then I have to stop when my chest and arms are too sore. Kind of like a ramping up in difficulty throughout the set. I guess that's why I voted for push ups.
Squats are similar to push ups, especially since I do full range of motion. But the limiting factor there is overall endurance. Mostly cardio, I think. I get out of breath and have to let my heart rate come down before continuing.