Once you figure out your riding style you can start to try and figure out the
best bike for your budget.
Of course there are the big names such as Specialized, Giant and Trek but there are other lesser known brands that offer great bikes as well such as Yeti and Scott. And once you pick a model many times there will be different levels of components for that model that will affect the price and performance of the bike. So once you have your price point and know the type of bike you want whether it be cross country, all mountain, freeride, downhill, 29er and so on, then it's time to go shopping.
One of the most well known brands of mountain bike manufacturers is Specialized. They make a wide variety of bikes for different riding styles and have something for everyone. If you want only plan on going downhill then check out the Demo. If you want some cross country action then have a look at the Camber for full suspension or HardRock for hardtail action. If you want something that does a little of everything then you may like the Stumpjumper which comes in 26 and 29 inch versions.