Hard to settle on just one.
I'm a big fan of 1095, O1 and A2 for camp knives. These sharpen fast, roll instead of chip, take insanely sharp edges, and are tough enough to not think twice about batoning, prying, chopping etc.
For folders I prefer H1 or DLC coated steels because corrosion resistance is very important to me. I'd like more DLC coated carbon tool steel folders, but those sprints are all small knives it seems. I really like the K390 on my Police 4 but the lack of corrosion resistance means I can only carry it about half the year. If I could get it in DLC it'd be at the top of my list for folder.
I'm also a huge fan of BD1. I've found it resists corrosion quite well, is very quick to sharpen, holds an edge about as well as VG10, and takes the best edges I've put on a knife. I love how sharp it gets, and how easily it gets so sharp. It wouldn't be my first choice for roles where serious edge retention is desired, but for typical pocket knife stuff I adore it.
If I had my way, Spyderco would produce lightweight versions in BD1 of all their popular Golden folders. I really like that steel.