depending on the type of mountian biking your into... for cross country, look into specialized, stumpjumpers. the stumpjumpers are top of the line for cross country. also, trek, garyfisher, rocky mountain, and cannondale all make decent cross country bikes. My brother has owned 2 rocky mountain bikes each for 5 years and they have held up amazingly. so i recommend rocky mountain and specialized. if your not into cross country mountain biking... well.. then i just wasted a lot of time explaining this.