That is a wide open question. The answer is: depends.
Have you already been riding? Have you been borrowing someone else's bike and are now hooked and know you want to pursue this?
What type of riding do you plan on doing? Do you just want to trail ride or do you want to do more serious, technical stuff?
My recommendation would be to visit several of the bike shops in your area. (not big box retailers) They can guide you, let you test ride different bikes, and get you a nice starter bike that fits you within your budget.
Unless you're past starter bike; they'd be happy to sell you that, too.
Some folks are super brand loyal, but as long as you're on a quality bike (THAT FITS YOU), that's all that matters.
FYI: I have a Giant Lust that I love. I also have a Cannondale, a Trek, another Giant and a Fuji. (the last two are road bikes) Oh, and hubby has an old Trek.