Worst product? Splitfire plugs. My bike does NOT foul plugs, because I know how to jet. Tried a Splitfire and fouled it in less than ten minutes.
Best product? My Ironman sprocket/Regina o-ring chain combo. Well over two hundred hours in abrasive Forida sand, and the sprockets still look new, while the chain has been adjusted once.
Most surprisng product? Forslick. A friend sent it to me and recommended that I try it, I was sceptical, but kept an open mind and tried it. Surprisingly, it workes as advertised. The forks had noticeably less stiction and were more supple and responsive feeling in ripples and stutter bumps. Too bad it was discontinued.
I have to give an honorable mention to MotoZ tires. I have the Tracula soft/intermediates, and they work exceptionally well in this soft sugary sand, and still cross over very well into harder surfaces. Being natural rubber instead of synthetic, they have one unusual quirk in that they take about a half a lap to warm up before they grip well, but once warmed up they are excellent. They also have exceptional life.