With condemned criminals there are far too many last minute reprieves (why can't the governor or judge make up his mind by 5pm?). Thus the drugs aim to first knock the criminal out, but still be perfectly recoverable if the order to stop is given. Then a drug to stop his heart quickly to minimize the time between injection and death.
Using this requirement, the guillotine is probably the best method. Everything is stoppable until the blade is dropped and a second or two later he's unrecoverably dead.