Lethal injection because it generally is the most effective and most "humane" way from my perspective. Other methods have more issues associated with them.
EDIT: It's my opinion so get over it. :)
In addition for those of you who don't understand how it works:
"The intravenous injection is usually a sequence of drugs given in a set sequence, designed to first induce unconsciousness followed by death through paralysis of respiratory muscles and/or by cardiac arrest through depolarization of cardiac muscle cells. The execution of the condemned in most states involves three separate injections (in sequential order):
Sodium thiopental: ultra-short action barbiturate, an anaesthesic agent capable of rendering the offender unconscious in a few seconds.
Pancuronium: non-depolarizing muscle relaxant, causes complete, fast and sustained paralysis of the skeletal striated muscles, including the diaphragm and the rest of the respiratory muscles; this would eventually cause death by asphyxiation.
Potassium chloride: stops the heart, and thus causes death by cardiac arrest. "
From Wikipedia, the free encyclopedia