Thinking back, Kaioken is probably my favorite DBZ powerup. It had multiple levels with drawbacks. It could technically be used by anyone who'd been taught it (although we never saw anyone but Goku use it, Yamcha, Tien, Chaoitzu, and Piccolo learned it IIRC but I could have misremembered...), and it felt like a desperate last resort technique instead of "Ah, this guy is strong. Guess I'll go Super Saiyan right off the bat."