Gougu theorem (he was first to find theorem before Pythagoras)
Any theorem about compass and straightedge constructions.
Euler’s identity
e^ixpi+1=0
The bellman equation, it should count as a theorem I think
Mine is more of a formula than a theorem: x = (-b ± √(b2 -4ac))/2 (the quadratic formula)
But in terms of formulae, my favourite theorem is probably pythagora's or the theorem that opposite angles in a cyclic quadrilateral add to 180º
The Chicken McNugget Theorem.
