I've been thinking about this & I simply can't come up with one answer. It depends largely on what factor makes a purchase particularly "good":
Gives the most pleasure:
Impossible to answer. I have a lot of guitars & on any given day any of them could be my favourite.
Led to the biggest improvement in my playing:
Indirectly, my first Warmoth strat - because it led me to BKPs, which in turn led me to recording clips for the forum, trying harder, learning to play things I didn't know etc etc. In other words, this forum (i.e. something that didn't directly cost anything) got me over a >10 year "plateau" in my playing :)
Led to the biggest improvement in tone:
So many things (not least of all, BKPs) have helped improve my sound. But probably the biggest improvement over what I already had was getting my Mesa Rect-o-verb head & Recto 2x12" cab (Crazy_Joe now owns it, so that head has the distinction of being cited twice in this thread... :))
Convenience:
That's one of the few criteria that is easy to answer: My Lehle 3@1 switcher. Makes guitar changes (& A/B tests) a breeze.
Biggest bargain:
hmmm, not sure. I've scored some nice deals over the years. The biggest saving relative to the "typical" price was probably my Jackson RR1T - got it new for $1400 (at a time when $:£ exchange rate was around 1.9!) :D
Rarest/hardest to replace:
Easy. Jackson Custom shop V. Since Fender bought Jackson, they no longer offer that body shape, so it's irreplacable.