You picked the worst possible day to ask which decks are best. Like, literally, there could not be a time when people are more clueless about the meta.
That said, if you are free to play and/or new to the game, you are not going to get to rank 5 anytime soon. Set a more realistic goal, or you are going to become very frustrated very quickly.
If you haven't pre-ordered any Uldum cards, you are going to have a similarly hard time. This is the most powerful set we've seen in a very long time, so if you're trying to get by with budget decks, Ranked is going to be harder than ever.
If you do have experience and some of the new cards, you're in the same boat as the rest of us, trying to figure out what's good.
Well.. yes BUT as with any new expansion aggro rules as people figure the meta out. Play an aggro deck for the first week and you should advance up pretty easily.
"I have seen worlds bathed in the Makers' flames, their denizens fading without as much as a whimper. Entire planetary systems born and razed in the time that it takes your mortal hearts to beat once. Yet all throughout, my own heart devoid of emotion... of empathy. I. Have. Felt. Nothing. A million-million lives wasted. Had they all held within them your tenacity? Had they all loved life as you do?"