RPGaDay 2023 #7 SMARTEST RPG you've played

 This would have to be GURPS.

Not only are the rules designed to relate to the real world as much as possible, using physics and real-world measures as much as possible, but they are also renowned for their historical sourcebooks that lean on historical research first and fictional tropes second. Their Cyberpunk sourcebook was based on research and consultations with hackers of the time, which got the company raided by the US Secret Service.

 Even their original settings focus on solid, realistic-feeling world-building. Their flagship fantasy setting includes not only elves and orcs, but also Christians and Muslims. Their Technomancer modern-magic setting tries to explore what would happen if magic had replaced atomic energy as the fuel of the latter half of the 20th century, while still keeping things somewhat familiar.

And the variety of sourcebooks is also impressive. There are even online tools that generate campaign ideas by making random selections of the available works. I've even run a campaign that was inspired by that process.