10. Superman - Superman #296 - #299
Being the most powerful superhero in the DC universe, naturally the idea of Superman retiring and the world adjusting to that retirement is a fascinating idea to explore. Equally fascinating, in this case, is focusing on how Superman himself would adjust to his own retirement. This is, after all, a guy who doesn't have an unhelpful bone in his body.
However, at some point he decided to at least give being Clark Kent full time the old Kryptonian college try. In Superman #296, Clark starts noticing an anomaly: whenever he changes into his Clark Kent persona, he loses his powers completely (caused by a villain who was fluctuating his powers, long story). He decides to test the theory by straight up abandoning the Superman identity altogether and becoming Clark Kent permanently. Even managing to finally work out a serious relationship with Lois Lane that lasted long after this story.
Superman even goes so far as to ignore someone in trouble because the police are already on their way and have it handled. But of course, the big lovable boy scout can't stay away forever, gets the anomaly worked out, but at least gets to keep his relationship with Lois.
But still the decision to quit being superman for almost four issues was a bold experiment on DC's part.