Question: Does ATC have the best midrange driver?

Audiokinesis makes the point well. With a midrange driver, or any driver, when the wavelength becomes less than the radius of the cone/dome is when the beaming begins. I don't now the math off hand, but this is why 4-4.5inches is about as big as you can get away with if you're going to try an get up to 6,000hz and still be able to cross as low as 100hz. If I was going to do I diy three way, which I hope to one day, the ATC and Skaaning would be on the top of my list. Personnaly, I wouldn't do the Accuton, for lack of internal damping. With the Manger's I've heard less than favorable experience from others with them-like "they were ok, but nothing spectactualar and I've heard betters stuff." I did hear there are some distortion peaks somewhere in the driver which may account for why they don't sound that great despite having some otherwise impressive specs. And they are $$$, but in defense easy to design for/with.

Ezmeral i've heard simular things about the Manger, even the word, "junk" was said. I just checked the Accuton specs. , is ferrofluid internal damping? One speaker designer says ATC, the other Accuton. I'm going Accuton, will let you know in about 3 months.

Internal damping, crudely, is the ability of the driver's material to absorb the standing wave/refelcted wave that forms on the front "inside" the cone. Someone else please say it better if you can cause I know I know what I'm talking about halfway here :) Aluminum and ceramic have virtually no interanl damping. This is all on the front of the cone; the ferrofluid damping has to do with the backwave of the cone-its different. Birotechnology.com and vmpsaudio.com both have good designer notes sections on the various cone materials you might want to look at short of the books for better explanations than mine. I know dunlavy slams most of those ritzy cone materials. If you get accuton to work great let me know. They are attractive looking and I'd like to design with'em, but the rational side of me still says the Skaanings would have a nicer sound in the end. But I'm no authority.

is this mid range used in the Proac 4 and or 5 or do those speakers use a "knock off" reproduction as someone recently told me?

The Proac 4 speakers used 2 ATC 3 inch dome midranges. The Proac 5 speakers used 2 7 inch Scanspeak midbases.

The best midbase I ever heard, is the JM Lab midbase on the Mezzo utopias and the Utopia speakers. The other midbase that sounded incredible, was the midbase on the Sonus Faber Extremas. Holy Moley did this midrange sound incredible. It was a custom made Skanning midbase. It was a Carbon Fiber Polypropelene midbase.