Jonathan Winters was the all-time whacko--like Robin Williams on steroids--though he was at his best in talk show situations. Youtube has some incredibly funny Dangerfield and Hackett schticks.
As for pure stand-up: I liked Jackie Vernon, an obscure but howlingly funny dead-panner who appeared on early Carson. Joan Rivers could be very funny before she became obsessed with talking about face lifts.
Speaking of dead panners , I always can find enjoyment in Steven Wright.
