Easy — Marvel Heroic Roleplaying by Margaret Weis. It’s the first iteration of Cortex Plus I liked (the other being Firefly) and I thought it captured the feel of the comic books very well. The rules were approachable, but could get a bit complex, with a lot of moving parts for what plot points could do, but all of that was easily stripped out, if needed.
Before that, I would have said DC Heroes by Mayfair. I liked the logarithmic progression of the stats, that they tied to time, weight, distance, etc… so that you had a good idea of exactly what your character could do. At the time, in the late ’80s, FASERIP Marvel Superheroes was too abstract (something that appeals, now), and Champions was out because I didn’t have access to a Cray computer or a few days to do character creation.