What is your best… and worst bike purchase? (Bonus points if it is simultaneously the same item). Ultimately trying to make myself feel better for my bad decisions. 
Me… Bought last year’s Epic Evo on clearance last year. Already had an XC and a trail bike so definitely didn’t NEED it. Have since have dumped a fair amount $$$ on upgrades… But man I really enjoy riding that bike.
What you got?
Bought a used S-Works Stumpjumper(2015?). Ridden it all of 75 miles in 3 years of ownership. But it allows me to say I’ve got a MTB(and a S-Works!)!
Power meter.
Oof, hard to call.
My new bike. Absolutely wonderful to ride. Looks great, seems fast, rides smooth, and has a few of the key features I wanted on a new bike (mainly aero frame, discs, tubeless).
But…I’ve learned that electronic shifting sucks. I hate it. And in reality I could have built an SL6 or something similar with my wants for less money, but I wanted a fresh start.
Bought a Fox 34 Stepcast for my XC bike to replace the 32SC. It was clearanced and told myself the 32SC would be a nice upgrade for my GFs bike. I haven’t raced it yet with the new fork so maybe I’ll be happier with my decision after this Sunday.
After many years of training and racing with power, and many years before that without power, I’m convinced it’s a waste. But I wouldn’t have known that without using one, so a catch 22.
Buying a $300 TT bike when I first got started in cycling/triathlon and I was totally broke. It started my addiction.
Why?
Best = anything the Mrs doesn’t know about.
Worst = anything I think the Mrs doesn’t know about but does, and uses it against me later.
Worst: cadence sensors. They don’t last enough. After replacing a couple, I realized it was better to buy a power meter instead.
Best: a smart trainer the first week of quarantine. It was heavily discounted, right before the demand boom. It helped me keep my sanity and overcome the dread I had for indoor training. I started doing structured workouts and my fitness improved a lot since then.
Another item to charge, zero tactile feedback, extra expense, more difficult to time shifts with pedal strokes. Mostly the tactile feedback and the lack of predictable timing.
Same issue on XX1 AXS and Dura Ace.
I agree. Electronic shifting buttons are great if you’re using Shimano road shifters. The feel of Shimanos brake/shift lever isn’t rewarding. Clicking mechanical SRAM or Shimano MTB way more satisfying and you can grab multiple gears at once.
My roadie is Dura Ace, and they are horrible. AXS MTB was better, but I still removed it. The mechanical X01 on my enduro is rhe best. Mechanical Shimano road is good, though I liked the Sram road mechanical slightly more. The XTR mechanical is barely one notch above AXS MTB, only because it’s mechanical.
All my answers are bikes. 
Best: my first adult road bike. A top-of-the-line Cannondale Synapse (High-Mod Red Etap Gen 1), all black and gorgeous, bought for less than half-price as a last-year’s-model-discount. Made me fall in love with cycling. Realistically, I’ve moved on and I’m glad I sold it to someone who will make great use of it… but I’ll remember it, and be grateful for it, always.
Runner-up: my third bike, a Specialized Pro Carbon Diverge that I changed to a 2x drivetrain and has been my only “road” bike for the last 3 years. I expect to keep this one for a long time, though it will eventually move to being a real gravel bike when my position improves and I add a good endurance bike and/or tri bike back in to the mix.
Worst: my second bike. 
A really nice hardtail I didn’t need and had no real use for, but I remembered years of horsing around as a kid on a BMX, doing jumps and tricks and whatnot, so I figured I’d find a use for it. Didn’t realize just how specialized MTB has become. And really had no use for it, really nowhere to ride it. Probably put less than 100 miles on it before I came to my senses.
Interesting. I’ve had 4 different bikes with electronic group sets and have never had an issue with predictably timing shifts with pedal strokes. I find that AXS road has better ‘feedback’ than Di2. Mostly due to the larger shifter paddles (especially when using thicker cold weather gloves).
When riding electronic I do sometimes miss the satisfying click of a mechanical shift. The sound of an electronic motor isn’t quite as nice.
Best - has to be the Garmin Varia.
Worst - a light I bought via a Kickstarter project. It’s bulky and difficult to fit on any of my bikes. In fact, I’ve probably used it once in over 5 years
Worst, a top peak bottle holder for the handlebars. It didn’t manage to last long. Closely followed by an early LED front light in 2003, which had the illumination capability of a candle in the dark lanes of my commute.
Best is probably just any of my bikes over the last 50 years. They’ve enabled me to go further and explore places and have adventures I would not have found / got to any other way. It would be unfair to single out any of the bikes over the years, as all have been favourites at one point or other.
Once more obvious stuff like head unit and power meter out of the way.
But would agree the radar is something that is massively underrated. I also miss it when riding without.
For me, the best use case is riding the TT bike. Where looking over a shoulder is much harder than the road bike. Just feel so much more aware of my surroundings.
Best has to be my garmin varia.
Worst is converting to a tubeless setup when living in an apartment
Worst bike related purchase has to be the service of the LBSs in my area. I have tried a few and maybe I just have unrealistic standards but I’d rather do it myself than get half assed work done on my bikes and being robbed in daylight for it.
Best related bike purchase has to be the Ride Shotgun Bike seat for my soon to be 3yrs old. The happy noises coming out of him when we ride down hills or on the pump track is worth a 1000 bikes.
this. absolutely this!
My best related bike purchase has to be a bike itself (hard to say which one though). If I was pressed to say an accessory I think I’d lean towards the sat nav/ bike computer (Garmin Edge). Worst accessory (actually a chain of accessories) was trying to find that perfect chain bath. IME they’re all brilliant first use or two but quickly deteriorate and fall apart. Switch to Drip Wax which would also be high up on my best list has been night and day with regards to chain cleaning.
Best bike-related purchases
Worst purchases
This is harder as I don’t think I have bought something that was outright bad, it just wasn’t for me. E. g. I got an endurance road bike for a great price as my second proper road bike (first “nice” road bike).