Best - white Celine triomphe shoulder bag, canvas with the beige monogram print. I really wanted a white bag (I think they're so chic!), but was concerned about getting it dirty (I'm not a careful person). Found a very happy medium with the triomphe canvas. The base color is an off-white but it's not all white. It was my first monogram bag as well, which I was a little hesitant about but I'm glad that the logo is more of a shape/design than words/letters. But I love carrying this bag! It's just the right amount of casual but also can dress an outfit down (which is important in Southern California, because not everyone likes to wear business casual like me

). It's my grab and go to bag in the spring/summer and I have to stop myself sometimes from reaching for this bag so often haha, try to show my other bags some more love.
And a small plus, I'm so glad I got it when it was $2200, because I probably would not have paid $3K for a canvas bag lol.
Worst - That's a bit tricky. I think it's a toss-up between a Chloe bag and the YSL denim puffer.
The Chloe bag was a beautiful bag, goatskin, nude, gold hdw, turn-lock closure, but it was a very spontaneous buy because it was on sale, 20 or 30% off retail price, and I didn't think about it and got it. I soon realized that the closure was too big and heavy for the soft goatskin and I could see potential creasing and sagging in the future and sold it before it could happen. (This was actually my very first luxury bag ever.)
The YSL denim, to be fair, I wanted a denim bag, but I never realized how little I would reach for it. It was also a spontaneous buy with the general thought of wanting a denim bag but not doing that much research. I wore it maybe 5 times in the past 2 years? Ended up selling it as well, with a bit of loss (but not too bad because it was basically new condition).
(Going with
@papertiger 's idea)
Lessons learned:
1) Don't get suckered into buying something just because it's on sale (by yourself or anyone else)
2) Do your research
3) Give yourself time to think about it to make sure it will most likely work for your lifestyle and aesthetic
4) Be patient
Random thought, I've realized if it's a spontaneous bag purchase, I usually end up regretting it or just stop reaching for it eventually. But if it's a spontaneous jewelry purchase, it always works out well and I love it for years! (perhaps my instincts are more attuned to shiny things

)