somebody spelled out what FAS-DPD stands for.

OTO is the simplest, is fairly reliable, but only measure TC and only goes to 5ppm (some kits stop at 3ppm). You can dilute it with distilled water to extend the range but you'll lose precision. Sometimes it's all you can do.

DPD usually uses a tablet. It CAN go up to 15, but there it bleaches out and looks like 0. It can measure FC and CC. I'm not a fan of DPD because it's as much a pain as FAS-DPD (actually, even more of one) but rarely do the kits go beyond 5ppm in what they can measure.

FAS-DPD is the most powerful, the most accurate and the most reliable. It takes a little longer than OTO, but can measure both FC and CC levels up to 50ppm! Is it the "BEST"? Well, by most standards, yes, it's the best.