Really this all depends on the price range you're looking at. If you want something that will last a few years I recommend paying more now for something you love rather than upgrading components later.
What terrain you're riding will play a massive factor as well. If you live somewhere fairly flat a hard tail or XC full suspension could be perfect. If you live in a place with lots of mountains rock faces an All-Mountain bike may be more what you are looking for.
In addition to both of those points you should decide on what wheel size you want. Basically there are 3 wheel sizes prominent in the market right now: 26, 29, 27.5 (called 650b).
26 inch wheels have been around for ages, they are the current standard in the industry and it is pretty hard to go wrong on them.
29 inch wheels are somewhat newer, though they have been around for a few years now and seem to be gaining an ever increasing level of support among riders. The basic idea behind them is that the larger diameter wheel will roll over things more easily than smaller sizes. This makes riding up hills much easier, especially on technical climbs. These wheels will feel quicker in pedalling as well since you are able to cover more ground with each revolution. The downside to these wheels is that the bikes wheel base will become a bit longer, leading to a somewhat more "sluggish" feel in tight corners.
27.5, or 650b wheels are a comparably new wheel style really only emerging into the main market in the past year or two. I have not ridden them so perhaps someone else would be better apt to explain the nuances, but from what I have read and understand from others who ride them is that they are a combination of the best parts in 29 and 26 inch wheels. Because they are smaller than a 29 the bikes wheel base can remain shorter and the geometry can be handled more similarly to a 26 bike (more slacked head tube leads to a more stable feeling ride overall). As well the wheel is bigger than a 26 so you will retain some of that "roll over everything" feel of a 29 inch wheel. This wheel will also be somewhat quicker than a 26 inch wheel on the flats, though a 29 will still be faster as it is a larger diameter.
Basically:
26: good on everything, industry standard for many years
29: great on ups and flats, faster than a 26 or 27.5, perhaps a more sluggish feel on tight downhill corners
27.5: "The best of both worlds" retains most of the good bits from both 26 and 29. (New to market)
From here you should be able to begin searching out a good bike for you, keep in mind a bike that is perfect for me may be awful for you, so always try and take the bike for a test ride. Here are a few things to ask before you buy a bike though
What am I using it for?
What is my price range?
What feel do I want?
Is it comfortable? (VERY important)
Do I want/need dual suspension?
With all that said I would recommend a few brands just based on my experiences with them or personal friends experiences.
Specialized — wide range of bikes, starting at moderate prices and going up to world campion levels.
Kona — huge selection of bikes at a variety of price points, may not be the lightest in the world but are generally well made and will last a long time.
Trek — Good quality bikes, generally light and well made. Never heard too much bad about them.
Giant — Good quality bikes, generally light and well made. Never heard too much bad about them.
Santa Cruz — Very well made, generally quite light and will last a long time. Price reflects this.
If you want more advice I would need to know more about your location and the terrain you plan to ride.
Hope this helps!