Captain Janice Lester before she went crazy and got stripped of rank only to blame her downfall on Starfleet's culture of sexism.
Garth of Izar trying to destroy Antos IV
The Talosians (seriously, those guys are desperate enough and powerful enough to attract starships from hundreds of light years away... you don't honestly think Starfleet is going to just leave them to their own devices?)
First Contact with the Bajorans, who at the time are a major spacefaring civilization and haven't met the Cardassians yet
First Contact with the Cardassians, who have only JUST discovered warp drive but are still powering their cities with fossil fuels and child labor
The Omega molecule
Garth of Izar trying to destroy Antos IV
The Talosians (seriously, those guys are desperate enough and powerful enough to attract starships from hundreds of light years away... you don't honestly think Starfleet is going to just leave them to their own devices?)
First Contact with the Bajorans, who at the time are a major spacefaring civilization and haven't met the Cardassians yet
First Contact with the Cardassians, who have only JUST discovered warp drive but are still powering their cities with fossil fuels and child labor
The Omega molecule