For me, it's the Ultra Combo Double system. It can really confuse your opponent, forcing them to guess.
For example, Zangief's Ultra Combo I was a ground move, but his second one was an aerial move (excellent for anti-air). Also, Cammy's Ultra Combo II, which was a counter move.
I would choose Marvel VS Capcom 2. Sure, it's a classic and still played a lot in Tournaments. But, it's also broken way too much (not that Ultimate Marvel VS Capcom 3 and Marvel VS Capcom Infinite are better balanced, as well. X-Factor and Infinity Storm are also pretty broken).
It can be about anything: Story Mode/Background/Lore, Fatalities, Brutalities, Animalities, Finishers, Fatal Blows, X-Rays, TV series, films (live-action or animated), etc, etc.
Being a very violent video game, of course that sooner or later we would have trouble sleeping. 😂🤣
If you have any question about why I posted a specific image, let me know and I will reply.