These are three major mountain bike brands.

yes!!

The ones sold by your favorite bike shop.

add Merida. OEM Factories..

Giant is who Trek hires , to make their bikes,

Merida does the same with Specialized, they own some of the company too..

any of those would be a great choice

I happen to like Fuji. I see a lot of the trek specialized and giants. I also see a lot of santa cruz scott etc loads of manufacturers available.

I started off on a Giant Talon and was hooked. After doing a couple single track races I was ready for a full suspension bike and I spent almost a year looking at bikes before I finally pulled the trigger on one. I was looking at the Giant Reign, Giant Trance, and Specialized Stumpjumper. After riding them all I like the Stumpjumper the most. When I went to purchase one they didn't have my size available so the bike shop really hooked me up. They sold me a Specialized Enduro 29er for same price as the Stumpy and I couldn't be happier. The bike is amazing!! Having a better bike immediately increased my confidence and speed. I would suggest riding as many bikes in your price range as possible and pick the one that feels the best. The only advise I would give is buy a better bike right from the start because it will make a world of a difference in how much you enjoy riding. You don't need to buy a carbon on anything like that but spend a little more upfront and get a better quality bike and you wont regret it. Here is my new bike.

Cannnondale?

Pick the one with the prettiest color -- i try to go with what complements my eyes the best

Bad Habit!!

LOL, shame on you to post this but got a belly laugh.

Giant makes bang for the buck bikes. Consider Talon bike in the under $1000 category.

But it would help if you specified budget limits.

All the above companies make good bikes. How you feel about the shop you purchase from is also really important. The suspension design between all three are actually meaningfully rather different. If you can, try to get significant test rides on each (preferably an actual demo on real trails).

What price range are you considering? What kind of riding do you do? What performance characteristics do you value? For example, some relevant questions for full suspension choices:
-How naturally supportive vs active do you want the suspension to feel under pedaling? Do you typically spin relatively smoothly in the saddle, or do you sometimes mash out of the saddle? Are you OK relying on compression damper adjustments for climbing, or do you need the bike to climb well with the shock wide open?
-Do you prefer a relatively linear feel to the rear suspension, or do you need more midstroke support?
-How concerned are you about decoupling the rear brake from the suspension?
-How concerned are you about frame stiffness? Do you side load the bike significantly when cornering?