Some gems that haven't been mentioned yet:

00s-early10s (some personal favs from a LOT of anime I watched in this decade - geass, death note, etc all classics but well known and regarded).

School Rumble: Yes it's highschool romcom, but perhaps the peak of the genre. Also a very early role for one of the greats, Ami Koshimizu as Tenma

Sayonara Zetsubou Sensei: The anti-high School romcom. I've always personally felt it was basically taking the piss out of shit like Negima and all the characters are fucked up. If you like Bakemonogatari you gotta go back and experience the original SHAFT kino.

Hidamari Sketch: the other SHAFT classic. This + SZS = Madoka. It's ume-sensei's work and the adaptation is very peak Shinbou style. As with most anime, there's gonna be some fan service but I've always felt in terms of "cute girls doing cute thing" shows this was the most earnest, since it really also tackles what it means to be creative (they're art students)

Kara no Kyoukai: technically movies but the best Nasuverse stuff and the only one I'd ever unequivocally recommend. Some great ufotable animation as well.

Eureka 7: surfing Mecha! Great music! A slow burn that you might not be able to get through the annoying protagonist for the first 30 episodes

Black Lagoon: doing crimes in the Pacific. It's very cool and explicitly anti Nazi.

80s/90s

Gunbuster: my eternal recommend, though caveat- it's Gainax with OVA fan service (nudity). Still one of the few truly great anime endings, it's something else when you finally get to the final scene of episode 6. You will feel feelings.

G Gundam: you probably caught some of this on toonami, but it still rocks pretty damn hard as a hot blooded mecha show

Initial D: EUROBEAT