best or most expensive? i am not so sure about that.
if you want to add cost then do a real watercooling system. this can easily add up to 30-40% of the cost for the system. you can easily run $5000 on a system like this.
instead of using little wuss fans use some deltas. some are $50 each. this can easily add up to $600 in fans.
get a mountain mods ascention case. you will need all that area for watercooling.
get a nice 55" 1080p oled display. this would be great for watching movies. $12,000
i wouldnt even bother with those speakers. get a high end receiver and high end speakers. depending on what you buy this could run $1500 to well over $30,000.
why bother with standard hard drives in such a system. ocz makes 8tb (yes terrabyte) drives which can be chained in raid for a total of 32tb. i know their small revdrive goes for $2500 so i know this new large drive is going to command a huge premium. then stack on multiples. i wouldnt be suprised if four of them would total $20,000 at this point.
why settle for average input devices? for an unlimited budget you can have an optimus maximus keyboard $1200.
it would be quite easy to spend in excess of $100,000 on a setup even without wasting money on such things as gold plated mice with diamonds.
a good virus though can fry your hard drives though making it useless. either that or a surge can fry everything.