Martin D35. There are few guitars I've played that would equal the satisfying intersection of sound, playability, and simple pleasure of owning and looking at it.
Guns I don't usually regret. But I'm glad I finally pulled my head out and bought a Beretta.
Knives - well, Spydercos were what I always wanted as a young man, but didn't think I could justify. I've had quite a few now, and don't miss the money. They are satisfying to own and use as well. There are more expensive blades, of course, and I have some custom fixed blades that are crazy expensive, but I carry and use a Spyderco and a SAK every day.