Worst minigame in the series?

[Firo Volondé]

Blitzball may be boring, but it's at least winnable (the randomisation is very clear, and explicitly stated in the tutorial) until you've got Attack Reels. After that there's not much point unless you hate item farming for Dark Matters.

Or like the Jupiter Sigil. It was a nightmare in my first playthrough, since I had neglected blitzball and had to play about 40 games in a row to get that damn gem. It can be mildly entertaining if played infrequently, though.

On topic, X-2 blitzball I hate the most, since I loathe any game you can't control yourself.