I don't have any experience, yet, with D2 or 420HC so can't comment on those.
My favorite slipjoint steels are CV, 1095, and to be honest, I've had good luck with 440C. Any steel needs to be heat treated right to make a good blade. I've had good luck with the 440C blades by Callimus, especially in the knives they made for Remington. I also have a late 80s hand made, fixed blade knife by a lessor maker in 440C that is excellent in terms of cutting edge, durability, and resharpening.
Mind you, I don't have hundreds of thousands of hours testing steels under harsh conditions. I just know what has worked for me over the years. I want something with a good edge that cuts well, lasts a little while, and is easy to get back to the same level of sharp.
In order of preference, I guess 1095 and CV would be interchangable in first and second. It all depends on what knife I'm getting it in. Obviously, if the knife I want is a Case, then it will be CV, but in another brand, 1095 could be the first choice. Third would be a good 440C. Mostly that is because I'm a traditionalist, I like the carbon steels, and you can't get a good patina on stainless.
I still have around a big MAXAM, lockback folder from the early 80s. It is ugly, big, and made in Japan (now they're Chinese), and made of 440(C I think, but could be A). It ain't nothing to excite a knife lover in terms of brand, materials, or looks. However, this thing sharpens up shaving sharp, stays sharp through a lot of work, and returns to shaving level with a few strokes on a ceramic stick or a fine stone. I used to carry it with the case duct tapped to my LBE in my early days in the Army. I never had cause to complain about its performance. Looks, level of sophisitcation, wow factor, those weren't its strong points. But it worked well where it mattered. Think of it as an old H&R top break shotgun. Not much to look at, but it sure does the job.
I guess I won't be getting any invites to the Steel Snob's Ball this year.
