Trek, Mongoose, GT, Apollo, DiamondBack, Giant or Haro for a $500-$650 hardtail, disc brake Mountain Bike?

These companies all have these specific requirements in the range of 500-650. I guess i am unfamiliar with the brands. Are any of these worth staying clear of? At the moment i am leaning towards a Trek 3900Disc, Giant Boulder Disc, or Mongoose Tyax Elite.
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