I would need to say necrons without much issue. The Halo Rings have an effective range, hence why they built multiple and all Necrons would need to do is destory one of these rings and they're fine and effectively sit their and make every sun in orbit of a Forerunner world go supernova.
Aginste covies or UNSC Necrons roflstomp everything.
In fact even if you pitted Forerunners aginste the Imperium they still lose, when you consider that a single strike force of space marines is capable of quite happily taking on an entire army and still coming back with little or no wounds.
W40K just wins aginste almost anything just because the lore is designed to be crazyily overpowered.