Buck of course. Huge difference in toughness for sure.
Cobalt, I just don't see it in my use.
Most of my knives are 420HC, 1095 or 12C27 hardened in the 58-59Rc range. Among these knives, discussing difference in edge retention or toughness is like discussing difference the 1/4 mile speed or fuel mileage among V-8 powered 1/4 ton pick up trucks. Or the difference between Sierra Nevada and Longunitas IPA.
I can tell an immediate and obvious difference between these steels and 440C.
I can tell an immediate and obvious difference between these steels at 58Rc and these steels at 56Rc (or lower).
But I can't tell any meaningful difference in use between these steels in use when they have similar Rc.
Knives that I've used with this type of steel hardened at the 58Rc (or higher) range include: many Bucks (420HC), many Opinels (12C27), a Schrade-Walden H-15 (1095), several Schrade USA (1095), Mora Companions (both 1095 and 12C27, the latter of which may be closer to 57Rc). I batton with the fixed blades (and the Opinels, occasionally) and do more wood carving that many folks do. I use them for shop use (including cutting down metal cans and flashing). If there are any differences in these steels, they aren't differences that show up in my use.
Knives that I've used with this type of steel hardened at 56Rc (or lower) include: several Case knives (420HC), Opinel Carbone (1086 at 56Rc), Victorinox Inox (55Rc) and Buck's 420J2 (55Rc, if that). At lower Rc levels, I think there is a bigger difference between the carbon steels and the stainless of this type. I find that soft carbon holds an edge better and hones up cleanly. Soft stainless of this sort dulls fast and is more prone to form wire edges when honing. IME, the Victorinox Inox and Buck's 420J2 are stand out clunkers.
If people want to bust Buck's chops for using inferior materials to meet a price point, their 420J2 slip joints are the right target. Just awful.
I would love to hear more about where you see 1095 as being tougher than Buck's 420HC. I don't deny it. Just noting that I don't see it in my use and I'm not particularly easy on them.