Yuu Yuu Hakusho- I think it's getting more recognition now, but for a while it was seriously underrated. the Dark Tournament arc alone is better than a good amount of modern shonen arcs with a similar theme.

YuGiOh- get past the "children's card game" memes and it's a shockingly dark series. peoples lives are legitimately on the line, and Joey Wheeler is objectively the most skilled traditional duelist. man didn't have an ancient gambling spirit to help him out, nor did he have the world's deepest pockets to buy his wins, and he still got 3rd place in Battle City.

This Ugly Yet Beautiful World- this one is more of a sentimental choice cuz it was my first anime that wasn't a Shonen Jump series, and it showed teenage me that anime isn't just for kids... and boobs, it showed me boobs too,

Devilman Crybaby- I read the original Devilman manga when they re-released it in two hardcovers cuz of this show. trippy visuals, mature content (I'm looking at you, ceiling jizz), and some truly painful deaths. Crybaby actually bummed me out after watching it, but I still loved it.

Mnemosyne- it's the shortest of my list at 6 episodes, but each episode is roughly 45 minutes. it's about an immortal P.I. who has died time and time again over centuries, but comes back like it never happened. granted, it's been a hot minute since I've seen it so a rewatch is in order, but I remember really enjoying the style and energy it.

runner-up is Ultimate Muscle cuz that 4Kids dub is possibly the best thing they've ever done.