So far on Switch 2, most of the technically impressive software has been from third parties with games like Star Wars Outlaws, Indiana Jones, RE Requiem, Pragmata, and the like, while Nintendo has been more conservative visually.

Looking back, who do you feel has historically done a more impressive job at pushing Nintendo hardware to its limits, Nintendo themselves, or outside studios?

Imho, RARE.

RARE.
Snes - DK2.
N64 - Jet Force Gemini.

CAPCOM.
GC - RE 4.

Wii and Switch.
Not sure...

NES - Sunsoft

Well, Breath of the Wild runs on the Wii U, so... easy pick.

With games like Donkey Kong Country 1-2-3, Banjo Tooie, Perfect Dark and Starfox Adventures it is easily Rare.

Capcom also is a strong contenter with Resident Evil 2, Resident Evil 4, Monster Hunter Rise and Resident Evil 9.

Monolith soft should also get a mention with Baten Kaitos, Xenoblade Chronicles, Xenoblade Chronicles X and their part in breath of the wild and tears of the kingdom.

Some peoples will not agree with me, but Pokemon Silver & Gold on GBC. They managed to put johto and Kanto in a single gbc card (Kanto is empty af, but still a entire region).

Usually the most "cutting edge" graphics on a Nintendo system will be from a 3rd party developer.  Nintendo usually uses a more cartoony art style that tends to be timeless.  I.e. the art is it not cutting edge, but if you are a retro gamer Nintendo games will be the best looking games on the system 10+ years from now.  For example, Mike Tyson's Punch Out now looks like a very impressive game compared to other NES games today.  On the other hand, Contra, Ninja Turtles, and Double Dragon on NES don't look nearly as good anymore.  Most Mario games still look pretty good IMHO, but if you compare the latest Mario game to the most cutting edge looking games today, then Mario won't measure up.

Nintendo is going for a timeless look instead of a cutting edge look.

IIRC that was just because of the increased cartridge size. If anything, their compression algorithm for Gold & Silver was worse than the one used in Red & Blue, as the people working on decompiling them found out.

Yup, it's Rare. When DK1 came out, our jaws were on the floor.

Agree... DK1 - DK3 looks like black magic on SNES.