3 head cassette decks

Tandberg and Revox are already mentioned, as are Teac's. If you don't mind spending some serious money, the Teac Z-series are awesome. Also the better Technic's decks are quite capable, as is the Yamaha KX-1200.
Best for it's money I ever had was an Aiwa AD-F800: sturdy, easy to use and very good-sounding. Ultimately I prefered my Nak ZX-9, but for the price paid the Aiwa was unbeatable.

I still have a Yamaha K1000, but don't use it very much. It has 3 heads, 2 motor direct drives and was one of the few decks that has DBX. It still sounds great.

The NAD 6300 should be in the mix, an excellent machine.

Believe it or not Sanyo made a serious deck back in the day, the 5370. You can buy them dirt cheap, IF you can find one. I still use it to make tapes for my car, it's a great sounding machine.

http://www.vintagecassette.com/Sanyo/RD5370

I second the NAD 6300. I have 2, both broken.