I'm always a fan of "best possible outcome" for 20, and "worst possible outcome that doesn't immediately kill anyone" for nat1.

If you're 2ft tall and trying to destroy an iron wall and roll a 20, you successfully scrape some iron off the wall. It doesn't just collapse spontaneously. If you roll a ~~0~~ 1, your attack bounces off and your weapon breaks. Luckily it didn't rebound and hit you.

It makes things more believable, however rule of cool is obviously above that. So if you can do a flip off a cliff onto the back of a dragon and convince it to attack the tarrasque for you before it eats you, and roll a 20 for all that, you better believe that dragon now considers you it's deity, and will die in a ball of flames, acid, ice, etc, in your name while flying headfirst down the tarrasque's throat. The tarrasque obviously dies from this, since it's so epic.

I guess it really depends on the stakes.