Three best things:
Any Shimano rear hub. I've had Saint, XT, XTR, 105. All have been dead silent and worked flawlessly. I've had to adjust the caps of two hubs, over the span of fifteen years of riding.
Any helmet that I have ever bought. Because I have destroyed three and avoided ending up in hospital with head injuries.
Gravity Dropper. Bought five years ago, works perfectly on close to zero maintenance. No slop, no creaks and no leaks compared to frequent problems associated with fancier, newer hydro/air seatposts.
Three worst things:
Hope rear hub. Not because it was poor quality, but because the loud, incessant buzzing detracted so much from the joy of riding in nature that I sold the thing within a couple of months.
Crank Bros Eggbeaters. Crappy engagement, and cleat ejection on rock strikes.
A road bike. Boooring.