The Mongoose XR Pro 29er and the XR200 26er are the best 2 from mongoose that are worthy of upgrades.
And the Genesis V2100 26er and Saber 26er are the best from Genesis with the Saber being the better of the two.
All models are all availiable from walmart. Heres a build of a genesis V2100.

Rockshocks shocks
Raceface cockpit
Sram X5 crank and shifting componants.
Dual mechanical disc brakes
Bike weights in at 29.8lbs
Aftermarket rims/tires