In short - anyone you want. Except Tau because fuck Tau.
...Not really? Can anyone reliably defeat the Tyranids or Necrons? If no, they lose. So fuck literally everyone but those two. In fact I'd say it comes down to a fight between those two.
Orks? They're currently spiralling up in a pretty big Waaagh! under Ghazzy at the moment in the fluff, and we've seen the insane shit they pull if they have enough time to escalate - planet battleships that tear apart planets to build other ships, Dreadnought-sized grunts with sophisticated arms and armor, competent tactics, the ability to conduct diplomacy, and a bunch of other shit that happens in the Beast Arises books - and according to a Harlequin from those books, those Orks are a pale shadow of the Krork at their high point. Apparently the Eldar somehow beat this at the height of their power, but that's long past, the Necrons wouldn't be able to beat them since they don't have C'Tan support anymore, and we've already seen the Orks as they are now give Hive Fleet Leviathan pause - it'd likely be a far sight easier for the Krork once they've grown enough, and if they keep spiralling up like they do so far, the numbers factor of the 'nids wouldn't really do anything besides give the Krork more cannon fodder.
Of course this is assuming no one headcaps Ghazzy before they really get going, which knowing GW is iffy but possible.