Hope Tech M4 F+R here. Why? Well because I had it good recommendation from forums such as this a few years ago.
They are a vast improvement on the OEM kit I had (Hayes Sole). But I can't claim they are the best thing out there.
Went for braided hoses too. I have these on my car and they make a difference, and I'd also want such a critical component to have a degree of extra protection (awaits flaming from those who point out why braided hoses can actually mask issues and so could be potential more risky).
I did originally have M4 up front and Mono Mini at the rear on a small rotor. However, even with bled brakes and new pads this combo simply didn't work on some of the faster descents. It isn't much fun going downhill with the rear simply not slowing you down
Called Hope and had a knowledgeable conversation with a chap there - excellent customer service IMHO to be able to talk directly to someone like this. In the end I decided to upgrade the rear rotor to a larger size and to put braided Tech M4 there too. Much improvement.
My only niggle with the Hope's (and this might be the case with others - I don't know) is that they need to be worked up to temperature to really make a bite. After a decent downhill section (XC downhill, not DH DH) the brakes really come to life. If only they could be that good when cold.