To acquire the 'Holorifle', 'Assassin Armor', sneak +10 and the 'Sierra Madre Reinforced Armor', a light armor with a DT of 18, you must do Dead Money. You can also become an extremly rich man.
Add to that breaking the Sierra Madre you then get the complimentary vouchers (every 3 game days???). Use them at the vending machine for an infinite supply of Stims, RadX, Rad away, food, chems, mines etc...hardcoe not a problem anymore.
If I had to chose 1 of the 3...easy...Dead Money.