Best...
Pioneer SP-BS22-LR bookshelf speakers, $100/pair at the time. Another audiophile gateway purchase. Seasoned audiophiles may scoff these are a revelation if you've never heard anything "audiophile" before and that's who I was at the time.
MiniDSP 2x4 $120ish, UMIK-1 calibrated mic, $125ish. The foundation of becoming an "objective audiophile" for me. Had a lot of fun measuring speakers in-room and in open-air and learning about the objective underpinnings of what we hear and what sounds good. Taking measurements and turning bad cheap speakers into decent ones with a few DSP tweaks was an absolute eye- (ear-?) opener.
Bose Quietcomfort 25/45 headphones, $100-$280. Yeah yeah yeah I know... Bose! The dirty word in audiophile-land.
But as Amir has shown these measure pretty well. And they sound fine. Tonally they sound great, not detail masters, but fine. As they should, with active onboard DSP. They help me sleep, they help me work, they help me on plane trips. I bought a bunch of refurb/used QC25's for ~$100 so I can have one at every desk and I have the QC45 for places where wireless makes more sense. In the long run I think they may help to preserve hearing as well... you can listen at a lower SPL, since you don't have to worry about drowning out background noise...
BIC America EV-15 "Eviction", $375 for the pair at the time. Yeah, these infamous guys. These were extremely educational. They sound better than their measurements would indicate. Massive power handling and high efficiency, to me, have a quality all their own. So much of the emotion of music is conveyed via dynamics. And these actually do measure well where it counts most -- through the vocal range. I am not going to tell you they are good speakers by ASR standards but you'll pry them from my cold dead hands. I'm also not going to tell you they're worth it at their current price, but $375 was absolutely a steal.
Paul Carmody's Overnight Sensations DIY Kit, $125 at the time. These speakers sound absolutely fantastic. I'll take these over just about anything commercial I've heard under $1K. Had a "spirited listening session" with a friend the other week with these and a $150 Sony 100W amp and jeez, that is a lot of fun for not much money.
Worst...
No real "misses" or audiophoolery purchases. I researched everything really well and was drawn to stuff with great price/performance. I guess in hindsight I slightly regret buying so much "mid" gear. Could have spent all that money one a smaller number of things that were more high-end. But all that "mid" stuff was part of the journey I guess.