Light is strong, and has no weakness, if you're maining just 1 element, this is an ideal element, so rerolling for fluffy is very popular. Dark also has no weakness, but just doesn't have good synergy between it's various parts, and so is not really competitive with other elements (yet).

In terms of strength, most people consider Thunder very high on that list, with wind pretty close by. Fire isn't bad either, water is probably a bit behind though (not as far as dark, mind you). The main issue with these is that you also need to have a decent team of the complementary element (fire for thunder, thunder for fire, water for wind, and wind for water) to cover for your weakness. If you have that, they are by no means weak, however, it's pretty hard to roll for 2 hundo...