These three are the best high performance edge steels that are AVALIBLE on production folders in 2018
Maxamet, K390, CPM Cruwear

It was difficult to narrow it it down to these three, especially with m4,10v and 52100.
BD1N and 4v are also favorites but there are currently no folders available in those steels.
Maxamet
This not a balanced steel, it's an extreme with extreme trade off's, but it has to be the strongest, longest cutting steel I've ever used. It won't hold a razor edge forever, but the edge it takes at high sharpness still makes me giggle.
It's raw aggression with precision.
If you bring the edge up right and feel it it screams at your finger tips!
Very enjoyable to touch up too, just gets sticky, with a real crispy edge, but let it dull enough and it's going to challenge you, especially if your using improper abrasives. I found Maxamet only works best if proper stones are used. CBN/diamond compounds truly make that edge POP!
Maxamet is not as stable as the other steels though. It's a big trade off for that "top speed"
K390
Or as Doc Dan says
"The M4 Killer"
I love that :D
They are completely different steels. More of a 10v compromise mod then a m4 counter part upgrade.
But whatever Bohler is doing to the steel and Spyderco is doing to the heat treatment.
It's KILLER!
Why?
It Holds a longer polished edge then M4, in fact I felt as though it holds that crisp polished edge longer then alot of steels which is surprising giving the alloy content.
Polished edges by nature are a fleeting thing.
also, K390 Sharpens great and has the edge holding closer to 10v.
I've noticed M4 will take a slightly more aggressive edge but doesn't hold that high sharpness as long. But M4 is a little more responsive to honing and touch ups; just crisps up faster but I keep it at a lower finish.
I'm really really enjoying k390, it definitely fits nicely in the gap between m4 and 10v with some unique properties.
The edge stable, way more then Maxamet, which is surprising given how much alloy is packed in the steel.
Maxamet definitely has a higher "top speed" the K390 though.
However, I find that like Maxamet, abrasive selection is king with K390 to really be able to shape the Apex the best. Diamond waterstones have shown to provide the crisper apex with polish, But touch ups on Spyderco ceramic works great too.
CPM Cruwear
This stuff might be the best all around edge performance steel. It doesn't have the raw power of Maxamet or the edge holding of k390.
But it has the best sharpening response.
This stuff WANTS to get sharp!
It has an aggression at the edge similar to M4 but slightly less. Yet it has alot more responsiveness to touch ups, honing and stropping. Which impressive, it's also a lot tougher.
I find this is an upgrade over it's cousin 4v in some regards.
Cruwear just has more BITE at the edge at the same sharpness and finish then 4V
I've attributed that to the added tungsten and chromium, but 4v is tougher even if it's a bit harder which is insane.
I think some of my best edges I've ever made have come off of cruwear. It's so easy to sharpen I don't even consider it a skill, it's just cruwear being cruwear so it excites me a little less :D
What's great about cruwear is it's less picky about abrasives, it's also less reactive then it's other tool steel brethren.
So while the other steels I mentioned will smoke cruwear in one or two areas, Cruwear seems to be the best all around edge performance steel that's available to buy on a production folder.
My words aren't the gospel but these are my observations over the past 6 months.
Shawn