Hot take but the Orks are the Necron's real enemy.

Let's say that all of the Necrons finally woke up on their Tomb Worlds. Every forum that talks about Necrons losing has two thing in common: all of the Necrons are still asleep and the nobles would bicker and not get along. When you have something like that, they're bound to just start destroying other things to prove who is the best.

They'd wipe the floor with the Tau. Despite their advanced Ion technology, a standard issue Necron weapon disintegrates opponents. 

The Eldar, they'd more than likely create more Anti-Psyker weapons in addition to their Pariahs. If not, then their reality bending weapons are more than enough.

The Dark Eldar are more or less the same, reality bending weapons

They're basically the Tyranids worst enemy because they have zero organics, living metal shells. Now they'll get some good hits in, but even the Necrons have this teleporting technology that kicks in when they're too injured.

Don't even get me started on the Imperium. Yes, they mass produce hulking machines, but they're more quantity than quality. Hell the Adeptus Mechanicus hates straying from the STC's they found, so innovation is pretty slow. 

The Necrons aren't even phased by Chaos, so they'll more than likely make Anti-Warp technology to fight Daemons, or feed them to the C'tan Shards.

The Orks are, even in their devolved state, the perfect soldiers. They mature in a year, instinctively know how to fight and fix, reproduce with spores so 1 can turn into 100 or more, and get enough together under a Big Boss and it's straight anarchy. Like, they genuinely warped reality because they believe throwing red on a car can make it go faster, which really happens. Who remembers when Ghazghkull got his head cut off by Ragnar? Well a Dok just put it back on and replaced some of his brain with bionics, wires, and some Squig sinew. 
They're immune to Chaos
The Tyranids don't even see them as worth infecting because they fight back
The Imperium is so worried that they'll exterminate worlds Orks died on
The Tau can't even convince them to the Greater Good
The Eldar, and Dark Eldar before the fall, already fought with them when they were Krorks so they know what's up 

Basically, if the Necrons woke up and took over, the Orks would be the greatest enemy they've faced next to the Old Ones.