I would think about the overall construction of the knife before I focus too much on steel. Just because a knife has a well known tough steel doesn't mean the KNIFE will be tougher overall than another model. An example here might be the Pacific, which has one of if not the most tough steels out there (H1) but in a linerless FRN handle you can only expect so much outright abuse before the handle breaks. By comparison a standard Endura with VG10 might have a less tough steel, but the addition of steel liners adds some amount of overall structural strength which will make the knife survive more abuse around the lock and pivot. So for me the "toughest Spyderco" is an overall toughness ranking, not just which model has the toughest steel.
Then lets think about how knives are most likely to fail. Lock type is the most commonly debated strength/weakness of a knife but I've seen a couple lock comparison videos where it isn't the lock that fails, it's usually the screws to the pivot or lock bar/stop pin that sheer off and allow the pins to rip out. I believe you can very easily build a friction folder that is as tough or more so than any knife with a lock, assuming you aren't stabbing with it. The pivot/screws and whatever kind of stop pin/screws stop the blade are most important (assuming that the blade itself doesn't snap off). This is an area where a Sebenza is going to be nearly bomb proof, considering how large their pivots and screws are.
The OP's titanium/Cruwear Native is definitely a tough combo overall. I would guess that the pivot and/or lock bar screws would sheer off in a failure before anything else. Maybe if enough pressure is put on the spine you might see the lock bar/tab round off or break but personally I thinks pine whacks are the most idiotic "test" you can do to a knife. Overall though that Native is one tough SOB, it's a good combo of strong scales, strong lock and strong blade.
That said I'd have to just throw out a few options, because it's all really speculation unless we've all pushed our knives past the failure point and have some kind of measurement to compare. Some of these I don't own, I'm just throwing out ideas...
Manix 2 - The CBBL is a very strong lock. This is another combo that I'd expect the pivot or stop pin screws to sheer off before anything else broke. The "stop pin" on these knives is also anchored by two screws on each side instead of just one like most knives have, so the blade has a very strong back stop to press into. On the standard Manix 2 it also has solid steel liners that extend to the edges of the handle so structurally it's a very strong overall package. I wouldn't be surprised if the blade tang itself is the weakest link in most failures on these models.
Southard - This is just a beefy little knife in every way...thick blade, big pivot, strong titanium frame lock bar. If anything the lock bar itself may fail, I've seen some frame lock bars fold outward when they fail.
Nilakka - This may be an unlikely candidate to some but I think this knife is near bomb proof aside from the obviously fragile blade tip. The handle/lock/pivot are crazy overbuilt and the blade is thicker than most so the pivot is extremely strong on this one. The lock bar is also among the thickest liner locks I've seen from Spyderco.
Vallotton Sub-Hilt - Just a tank built knife overall. It's thick everywhere. I would rank this knife up in the very top tier of toughest knives Spyderco have built. The blade is a sharpened pry bar and the liner lock is super thick.
Gayle Bradley 1 - Much like the Vallotton, it's just thick all over. It also benefits from a tough standard steel option.
Techno - Short/thick/titanium, you can only apply so much leverage to one of these which will limit how much abuse you can put on it.
Chinook - Especially the current version with the Power Lock, I would expect screws to sheer or the blade to snap before anything else.
Shaman - This one is pure speculation since I haven't even had one in hand yet, but the overall package appears to be very tough. The Cruwear/4V versions especially will probably take a beating (from the ones of us who dare to abuse such a sure-to-be collectibles).
I'm sure there are many many more..
SHARPEN IT LIKE YOU LOVE IT, USE IT LIKE YOU HATE IT