Skylab
Skylab was an enormous scientific accomplishment.
It benefitted from a lot of room, being the first real science outpost (everything to learn).
It also contributed to our knowledge of the Sun and the Earth.
As for humans in weightlessness, it helped explain why there is space sickness, why too big a volume is not good, it was also improvisation (solar panel repair). After Skylab, 0G effects (like loosing small things) was better understood, the need for orientating ourselves (what is up, what is down) too, etc....
Compared to what had been done before, Skylab was a giant leap. But since, there are only small steps.