79. X-Men vs. Cassandra Nova
While Venom made a notable entrance, Cassandra Nova was even more impressive, as she first showed herself by tricking a relative of the original Sentinel makers to give Nova access to the Sentinels. At which point Nova launched an assault on the mutant haven of Genosha. The small island nation had 16 million mutants. When the Sentinel attack ended, it had about 100.
And that was not even the most dramatic of Nova’s plans!
Her next step was to take control of the body of Professor Charles Xavier, who it turns out, was the twin brother of Cassandra Nova! She died in the womb, but managed to survive as pure malevolent energy, until finally becoming corporeal.
Now in control of Xavier’s body, after a bit of mischief at the X-Mansion (like breaking Beast’s spirits completely) she travels to the Shi’Ar Empire, where she quickly takes over the entire empire and lays waste to most of it and sets her sights on Earth!
Luckily, the X-Men are able to defeat the invasion as well as trap Nova in a body where she can (seemingly) do no more harm.
A great storyline by Grant Morrison and various artists (most notably Frank Quitely).