Apollo 9 was an exceptional engineering flight. The tests provided for a fair bit of knowledge that was relied on for the Apollo 13 recovery. Much of the emergency actions that were required by the O2 tank failure came from the earlier data and system testing. Apart from the LMP's "chicken salad"...
Gemini 4 was also important, for initial rendezvous, Dave Whites EVA and the discovery of the same problem that had nearly killed Alexi Leonov on Voshkod-2.
Ed White just before overheating trying to get an expanded suit back inside
Alexi Leonov on the first ever space walk, before having a major scare trying to get back into the blow up transfer chamber.
Back when testing was done in real time without the HR, CRM, FAA, EASA, ICAO PC bandaids
That was a life well lived. Vlad, take note.