situp is the least helpful exercise.
if you want to maximize your strength training effect use compound exercises that works the main muscles of the body.
As suggested by do pushups, squat and I would add also pull-ups.
And if you want to strengthen your belly do the plank exercise and when it's getting easy for you, go for the extended plank variation (a better exercise than situp)
RE: 30 Day Sit Up Challenge - Day 1