If you're willing to try one that isn't a flipper, given that you're specifying hard use as one of your qualifications, I can't recommend the Buck 110/112 highly enough. I have my dad's 40+ year old 112, which he wore, used, and abused every day, and which I've carried every day since he passed. It's had the blade replaced twice. It's been used as a prybar, screwdriver, stake, and even as a knife once or twice. It still locks up like a new knife. Buck's warranty service is outstanding (at least on their US-made knives); both times the blade was replaced, we told them he abused it and would pay for the repairs. They fixed it at no charge anyway.
I'd try to find one of the models with the S30V blade. They're not too common, but you can always order it from their custom shop. On the other hand, while it holds it's edge longer than the 420HC, it's tougher to sharpen, so if you think you'll need to sharpen it often in the field, you might be happier with the 420.
If the speed of deployment bothers you, speed sheaths are available (the knife sits in the sheath partially open, and a pivot pin locks the blade open as you draw it). I got a very high-quality handmade one for around $30 for my dad's knife, and it works great. Faster than any flipper, because the blade is locked open as soon as it clears the sheath. Obviously that doesn't work for pocket carry, though.
My favorite heavy-duty flipper is my Kershaw Rake. Don't know if it's still available or not, but that D2 edge takes a beating, the lock is probably the most solid liner lock I've owned, and it's a good working size (I have big hands too).
Any of the other folders I've owned (though I like them and use them), I wouldn't consider for hard use. They may have finer edges, better steels, smoother actions, or whatever, but those are rarely related to durability. Not that my collection is anything special, though, so I'm sure there are plenty of other good options; those are just the ones I'm familiar with.