Voted for Eternity and Pulverizer, since I consider those to be by far the best ones, and apparently the community agrees with me, based on the poll at this time. Pulverizer seems to be the hardest one to win for some reason, so it might be wiser to go with that, though Eternity is easier to play with, since it only needs one elements, not 2.
FWIW, I picked Druid Staff when I did the quest, because I was playing Monolife at the time. It did alright, but when I wanted to make scaredgirl's rainbow deck, I was decking out a lot because I couldn't farm an eternity for a long time. Plus, druid staffs are easier to farm; more AI decks use them.