I design bikes for a living and am constantly up to my elbows in stuff like this.
Any bike you find in a BIKE SHOP will do well, and most BMX bikes of similar price are specced very close to each other. Don't believe those that say a Haro hub is better than a GT hub, or that a Mongoose or Diamondback sucks. They really don't know other than what their buddies tell them.
By the way, the 2008 Dew Cup Champion won on a Mongoose.
If the bikes you are looking at are similarly priced, the Mongoose will probably have slightly better specs, the GT will be next, and the Haro will be lowest specced. That's just the way it is until you get to high end bikes, especially BMX racing bikes. At that point Diamondback is far ahead of the crowd.
If you are interested in a bit of history, it was Mongoose that introduced the first BMX production bike and Diamondback that introduced the first BMX bike with an aluminum frame. Thats a legacy that Haro and GT can't match.
29 years in the industry