Which exercise do you see done with improper form the most?

Squats and Deadlifts, without question.

Too often for back squats (for example) I'll see people a) squatting on their toes, not keeping the weight in their heels b) not reaching full depth where the crease of the hip is below the top of the knee c) at the bottom of the squat/on the way up, their knees completely collapse underneath them, rather than having their knees track in line with their toes. So much wrong.... And with deadlifts... rounding of the back, not using their posterior chain to lift the weight, not keeping their core engaged, rounding of the shoulders, chest down... I could go on.

Squats and lunges. I try to remind people not to flex their knees over their toes when doing them but they don't listen. Also when some people squat they don't drop their butt and tend to bend over.

Sit ups and push ups are also right up as the most abused exercises. I of course, never fudged on any exercise or PFT.