Here are my ratings for each Element's nymph.
Aether - Not very good. Because of it's weak ability, it's only useful to provide damage in a rush, but Aether doesn't really have any Rush cards.
Air - Not very good. It's too slow against a rush, too expensive against a denial, and too frail against a stall.
Darkness - Not very good. Its ability isn't something you'll need more than once or twice.
Death - Not very good. Aflatoxin is better, but not even Aflatoxin is any good.
Earth - Not very good. Too slow against a rush and not helpful against a stall or denial deck.
Entropy - Not very good. It's way too frail and can only be good against something with no CC, but it's too slow against something with no CC.
Fire - Not very good. Due to its ability and stats, it could only be useful against a rush, but it's too expensive in an anti-rush deck.
Gravity - Not very good. Its ability is only decent against rainbows, but rainbows have CC to easily kill it.
Life - Not very good. Adrenaline is for rushes, but this is far too expensive for a rush.
Light - Not very good. Slower than Luciferin, hardly does any damage, and its healing isn't as powerful as Miracle.
Time - Not very good. There's no need to want Precognition more than once. Hourglass is better for draw power and GotP is a better attacker.
Water - Not very good. It's an unreliable and expensive way to summon something that isn't even useful.
Conclusion: Stop complaining if you don't have any nymphs. They all suck.
Some people might say that is crazy but I actually agree. When was the last time you have seen someone use a nymph in a deck and have it be game changing? I for one rarely use my nymphs, I have them for more of a collection/show off purpose. In PvP, nymphs have a very small value. However, when it comes to submitting arena decks, I find that nymphs are commonly or somewhat commonly added to decks, especially gravity decks. They don't really make the decks better, but they don't hurt them.
As for the poll, gravity is definitely the best, and there are a few close behind it (entropy, air, life (i find life nymph to be very underrated))