Meiwa is sweetest. Fukushu and Marumi are more tart but still sweet. Nagami (including Nordmann) is more sour but still rather sweet if you wait until they are fully ripe. Nagami types are a bit more astringent and sometimes have a soapy taste, but they are still fantastic and one of my favorite citrus. I think all four are really good, and anybody who likes kumquats should try them all.
One important thing I've noticed is that for fukushu & marumi, the smaller fruits are better. As they grow large, sometimes to the size of chicken eggs, they become more bland. The smaller fruits are better for these. While for Nagami types, because they are so sour, the larger fruits are sweeter and more preferable.