62. Supergirl vs. Anti-Monitor
The times were looking pretty bleak for the heroes of the multiverse about mid-way through Crisis on Infinite Earths. The Monitor, the big guy that was helping the heroes against the Anti-Monitor, was now dead.
In his death, however, he managed to protect five Earths (coincidentally, the ones with all the coolest superheroes on them). However, that respite did not appear to be lasting too long, as the Anti-Monitor was beginning an assault on these last five Earths and if the heroes did not so something quick, they would not even have a chance to come up with any counter-attacks against the Anti-Monitor.
The situation was getting rough, and the relatively tiny heroes were tossed, if not for the courage of Supergirl, the heroes would all be lost.
Luckily, Supergirl decided to make a stand and take on the Anti-Monitor all by her lonesome, and in doing so, she allowed the other heroes to escape and plan for another day. She did not manage to KILL the Anti-Monitor, but she sure managed to make him know that he had been in a fight. Sadly, he also killed her in the ensuing battle.
The epic fight, masterfully executed by Marv Wolfman and George Perez, led to one of the most iconic covers of the past twenty-five years.