Front derailleurs are dead as aurochs, so XX1 is now "smart money". You can debate me in the comments if you disagree, but IMO the performance advantage over any other system out there right now is huge. Hopefully we'll see an X.9 level version next year.
Note that this build applies to any wheel size (26/650/29).
Smart Money 2013 - $4000
Frame: WW custom hardtail (full suspension: add $900, includes Fox CTD rear shock)
|Fork||Fox RLC (taper or non, 15 or 9mm axle)|
|Handlebar||Easton EC70 or Haven Carbon|
|Brake levers||Shimano XT (choice of rotor size)|
|Wheelset||ZTR hubs with Crest/Arch/Flow rims (your choice)|
|Saddle||WTB Laser V pro|