For Me I will say "my External Hard Drive", it is the most important and expensive piece of gear that I have in my rig. Everything else can be easily replaced. Most of my music are promo reviews on MP3 formats so I cannot revert back to a CD for re-ripping. Even though the ones that were ripped can be re done, the time and expense to do so is phenomenal.
Ahhh...there is the issue.
In the original post you said most "important" component, but in your own answer you respond with your most "irreplaceable" component. Two entirely different things.
Read my first sentence again! I stated "what is" then went on to explain "Why".