I wouldn't buy a expensive MTB because, as the term implies, your going to be trashing it off road, so why pay $1,000 or more for something your going to break...eventually, and sooner rather then later, as compared to a road bike. I have MTB, though it only has about 8,000 miles on it and about 1,200 were off road, but it was those 1,200 miles that beat the thing into many repairs, and this is XT stuff!
I have a cheap Diamondback Sorrento that I have put through absolute hell and it keeps beggin' for more.
As far as top brands go, I have heard the best reviews on:
Trek Kona Yeti Cannondale GT
(Incidentally, I have an GT TT bike and I can tell you that it is awesome.GT Fanatic is on to something because they have made some great bikes. I'm particularly fond of the late 90's "triple-triangle" frames.)
Let's take it a little slower. Before you pick your bike... You need to find out what size bike you need. You don't want a bike to big. You will come off your pedals at a fast speed and crack your cods on the top of the frame. I have a Giant. It has all shimsno xt components with rapid fire shifting. You can find an older one for around $3-400. It's all you need to start. If you like it, and it's something you want to get further into.... You can upgrade or custom build one to suit your type of riding. Hope this helps.