I would say vampire right now, but it is very likely that sam skull/grace will be nerfed in the future which makes vampire kinda weaker. If you are f2p then my advice is to always save your gems for the NEXT meta, not the current one.
if you desperately need a top tier deck, just go for the BEWD deck... guess it's the cheapest deck in the top tier right now... or maybe bujin if you don't wanna spend some dollar to a kids game.