X-Men ’97 (Season 1)

Skippable in the sense that it sits outside the MCU's timeline entirely, and it is one of the best things Marvel has put its name to. It is a direct continuation of the 1992 animated series, picking up where that ended, with much of the original voice cast and the same theme.

It does not require the original, which is the surprise. The first episode establishes the team, the school and the situation cleanly, and the series is written so that a newcomer follows everything while anybody who watched the 1990s version gets thirty years of payoff.

What distinguishes it is that it is genuinely serialised and genuinely bleak. The season commits to consequences that a children's cartoon would normally reverse: a massacre in the middle of the run that the series does not undo, a relationship that ends badly, and a finale that scatters the cast across time. The politics are the ones the comics always had and the 1990s show could only imply.

The animation is a deliberate update of the original style rather than a replacement, and the writing is tighter than most of Marvel's live-action television. Ten episodes. Watch it whenever; nothing in the films touches it.