That depends. If you want to fight the Fake Gods (FG, if you don't already know), rainbow (control) is one of the strongest decks out there. However, there are some other decks that are just as good. If you want to play L0-3, then most decktypes work, but some decks are stonger against some enemies than others (obviously).
It's still a young game, and there have been at least two 'uber-decks'. Mono-Aether was
the thing in the summer, but then Phase dragon was made un PU-able (Parallel/Twin Universe, a card which copies another card). Rainbow is big at the moment, but that may change with a rumored Sudial nerf coming soon. I think there was another, but I'm not sure.
If you want to find decks to play, go to the wiki at
http://elementsthegame.wikia.com (
http://elementsthegame.wikia.com) (may change soon) , or the Decks section of the forum.
I don't do PvP much, but that's because I don't like the system it uses, but that may also change in the future.
Zanzarino (the developer) has started making new cards within the last week or so, and has let chriskang work on sone of the AI coding, so it looks like work has restarted on the game (Yay!)
Really, just stick around and see - that's the best advice I can give you. Hopefully someone else will be more helpful.
PS - Sorry, too many parentheses.