I have to go by the different eras and time periods on that one. Because back in the mid 90s when I got my first Spyder with GIN-1 I thought it was right on with serrated edges. But then when I got my very first JD Smith model in the late 90s with ATS-34 I had thought that was a premium steel and one I could really rely on>> I still have a couple of ATS34 Spyders. Even though it wasn't long after that until Spyderco released ATS-55 of which I still have quite a few models made with that steel and I think it's better than ATS-34 in PE & SE both.
OK let's fast-forward a bit. In 2005 when I got my Burgundy Caly Jr with ZDP-189 I thought for sure I had hit the pinnacle for a high end blade steel for plain edges>> but it wasn't too long after that I got my first Spyder with S90V which I liked better for a few reasons especially in PE.
But in the past 3 years my personal favorite has been the ultra-reliable M390 even though I also like XHP for different types of blades>> I'm hoping to get a serious Spyderco model with Teeth with XHP because I think that might just be the best SE blade steel out there.
But at this present time the winner will be M390.
Because I use my M390 Millie quite a bit throughout the week. Although I admit that there are 3 supersteels I haven't gotten to try out yet that I've heard that rate higher than M390. But it will have to be a real "Barn Burner" to beat out M390 IMO