It's a bit hard question, a lot of old anime shows I watched recently and can't exactly tell whether they're from the 80s or 90s

I just watched violence jack a month ago

However, I watched fist of north star back in the early 90s and it was the best thing I've seen, but I recon its older than that

As for 2000s, there were many great things like code Geass, death note, Fullmetal alchemist, guren lagan

When it comes to the last couple of years, I'd say Dr.stone

Hokuto no Ken was pretty cool. It's originally from the 80s, but it got many seasons, even as late as 2016 or something like that.

Anime kind of exploded during the late 1990s and early 2000s, there are so many anime made around that time that it's hard to name all the great ones. Among the generally overlooked ones, I really enjoyed Gallery Fake (2005) and Master Keaton (1998).

90s - Evangelion

00s - Chrno Crusade

10s - Kabaneri of the Iron Fortress

20s - Not sure yet, maybe To Aru Railgun S3, 86, Vivy, or Evangelion 4.0.

80's - Gera Gera Booth Monogatari
90's - Dragon Ball Z
2000's - Fullmetal Panic!
2010's - One-Punch Man
2020's - Mushoku Tensei: Jobless Reincarnation