That's an easy one. Voltage is really the only dangerous one on that list. Too much will fry your cpu and you will have to buy a new one. When your comp gets too hot, it will shut itself off to save itself. Too high of a clockrate (speed) will crash your computer before it boots, but wont permanently damage it.
There is no answer - because all of them are inter-related and interdependent. more clock=more volt =more heat. You want one - you get the others. welcome to the world of electrical engineering laws and realities.
Have a proper cooling solution, allows more volt, which allows more speed. Therefore in VERY simplistic terms, heat is your weaklink that you can control.