Who makes the best and least-noisiest pump?

Cant comment on the Eheim pumps. I have a couple of their canister filters and they are pretty quiet stuff. I also have a Mag 9.5 pump and its not quiet. I wouldnt call it noisy either but its not silent. I was impressed with the Mag pump though. Holy water movement LOL.

I don't like Pentair Quiet One pumps. I bought a Quiet One 4000 pump (1027gph) about a year ago, and IMO it stinks compared to another that I've tried. Rio Plus 2100 (675 gph) is the better powerhead I've tried. My Rio Plus 2100 can shot water clear across my 10 ft room. I've never had any problems priming it when I'm using it to push water though 50' ft of 3/4" tubing. (Which is something my Quiet One pump won't do). I would think that a pump rated at 1027 gph would be stronger than one rated at 675 gph, but it isn't. The lesser rated Rio Pump blows it away. Maybe I just got a lemon Quiet one, but who knows. The Rio ain't an external pump though, if that matters.

Aquaclear also has some nice budget powerheads, but they are not as quiet or as durable as the Rio Plus.

Eheim's are dead silent and very reliable. They're a little more expensive then other pumps but well worth the extra money IMO.