I can´t imagine things getting much "tougher" concerning Spyderco folders than my Delica comboedge sabregrind and the Endura sabre grind (which I sold just a few weeks ago to get a Pac Salt SE instead):
Almost no distal taper, thick sabre grind, very stout tip, undestructable FRN, steel liners, a lot of steel in the back due to the lock, closed back generally, VG10, and last but not least backlock... that´s "toughness" for me!
My Pac Salt SE feels very tough too, H1 should be even tougher than VG10. If you push it to the extreme, the linerless handle must be a bit "weaker" though than the linered of Endura and Delica of course, but I don´t think that is of real practical relevance. The tip of the Pac Salt is also a bit "weaker" just by design (not concerning the H1 steel!) being chisel grind. Again, most likely no practical relevance though.
Top three going by pocket-time: Endura 4 in VG 10/Micarta-scales; Stretch 1 in VG 10; Endura 4 in HAP 40