The best, strangely enough, was probably the FIC VA-503+ that I used back in the Super Socket 7 era. That board would run any Socket 5 or 7 CPU ever made, would run on SDRAM or EDO, and overclocked anything that could be overclocked. I had it for 12 years, long past the point when it was viable as a main rig, before I passed it on to someone who was working on a vintage computing project. For the last several years I had it, it was running a K6-III+ at 600 MHz with 512 MB of SDRAM.
The worst? That's easy. A Leadtek board from about 2002, WinFast something or another. Worked great for a while. One night, I was smelling something burning in my den, and I discovered it coming from the computer. I opened the case up, and the next thing I knew, the caps around the CPU socket were literally going POW! and shooting sparks, one after another. I've never seen a board die so dramatically. I went to the utility box and cut the power to that outlet, and the caps quit blasting. It wasn't the last time I ran into bad Chinese caps, but it was the worst by far.
The only good thing about that episode is that, amazingly, neither the CPU (Athlon XP 1600+ at 1813 MHz) nor any of the expansion cards were damaged. I fired up the old machine with its trusty VA-503+, send Newegg an RMA, and had a refund by the next morning. I guess my story got their attention!