What's your greatest purchase...

Nice sentiment French_fries. I agree completely.
As for equipment, I purchased a pair of Meadowlark Kestrel Hot Rods that still blow me away with their wonderful musicality. Amazing speakers for the <$500 I paid a few years ago. This site turned me on to the now defunct brand.
Another item is the Resolution Audio CD-50 I got so cheap I won't say. With this player I hear truly beautiful sound from a digital source. Actually it came from ebay now that I think of it but I learned about Resolution Audio on Audiogon.

Lamm ML2 amps.

Many years ago my TEAC X-10 reel to reel. Loved it. As of late an integrated tube amp, my first tube component. Coupled with the VSA jr4's, magical.

A CJ CAV50 integrated and DV2B CD player both from Audiogon.

My biggest purchase was a Krell KAV 280CD player new at a good price. My biggest savings were on a Wadia 64 D/A, (used in flawless condition) at about 15% of retail Second to a CAL DVD player from reviewer (when he heard repair places were out of CAL parts) at about 13% of retail, 4 months old. The general ability to move up without the retail (and associated) strain.
But what French fries describes puts the icing on the cake as my experience has been of fellowship.