Most important equipment
My first thought was my main speakers. They are a pair of classic ESS Heils that would be nearly impossible to replace, though I think the Mini's might be able to do it.
My Harmony 880 might be a close second, the best remote I have ever used. Seriously, this makes the remote juggle a thing of the past. Highly recommended.
Receivers, TV's, DVD players etc.. are in constant flux, new better items are always being introduced and upgrading takes on an "every several years" cycle. If something breaks, just about anything you buy to replace it will be better. Oh yes, I recommend that you don't buy very expensive AV furniture, it always seems to need replacement every time you upgrade your TV.
What becomes your most important piece of equipment, in the end, is your ears, eyes and brain. All of this stuff that we buy and lavish many hours over tweaking, playing, listening to, etc. is there to recreate the best sounds and images to entertain our brain. Without our health, and properly functioning senses then all of our precious equipment becomes so much junk.
Protect your ears, use hearing protection when using loud equipment. Use eye protection when appropriate. Keep your kids from killing their ears with their Ipods. Simple precautions will help you to enjoy your equipment for decades to come.
Jim