Of all of my bikes and such, the best, smoothest, easiest-to-be-precise derailer for me is my Campy NR. Smooth like butter.
Strange as it sounds, I kinda like the plastic fantastic Simplex, and wish I could trust them more.
That said, IMO, the best "daily driver" derailers are Suntour anything. Given the choice, I'd pick Suntour mechs over nearly anything. They're pretty, work well, and are bulletproof.