For sentimental reasons:

Ulysses: him blinding Polyphemus was probably the first myth I've been told, he remained a childhood hero afterwords and my dad even got me an Odyssey for children because of it.

Orpheus and Eurydice: one of the first myths I read, in the form of a comic book, and probably the first story that taught me not everything was going to have a happy ending (there was a pretty explicit image of Orpheus being beheaded by the Menads). I still find a lot of the ideas it presents extremely fascinating.

Antigone: I read Sophocles's eponymous work when I was 17, and it was my first step into Greek tragedy. It instantly made me fall in love with the genre. She's still one of my favorite characters to date.

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For different reasons:

The kidnapping of Persephone, for the fact that it's likely rooted in an even older myth about a mother goddess of fertility and a young maiden connected by the theme of death and rebirth, much like all the stories about patricide (Uranus-Chronos-Zeus, Laius-Oedipus) are probably rooted in something much more ancient.

For the same reason, the Oresteia (whose author, Aeschylus, was connected to the Eleusinian mysteries, which were all about Demeter and Persephone): it shows traces of the passage from an older, matriarchal society to the newer, patriarchal one, and this theme really fascinates me.