Every fandom has an iceberg. Here is anime's, from surface level to the abyss.
Level 1 — Gateway (everyone knows these): Dragon Ball Z, Naruto, One Piece, Attack on Titan, Demon Slayer, My Hero Academia
Level 2 — Popular but requires seeking out: Fullmetal Alchemist: Brotherhood, Death Note, Cowboy Bebop, Steins;Gate, Code Geass, Hunter x Hunter (2011)
Level 3 — Enthusiast level: Vinland Saga, Mob Psycho 100, March Comes in Like a Lion, Ping Pong the Animation, Monster, Odd Taxi
Level 4 — Deep cut classics: Legend of the Galactic Heroes (110 episodes of space opera), Berserk (1997), Revolutionary Girl Utena, Neon Genesis Evangelion + End of Evangelion
Level 5 — Art house anime: Tatami Galaxy, Mononoke (2007, not the Ghibli film), Paranoia Agent (Satoshi Kon), Serial Experiments Lain, Haibane Renmei
Level 6 — Challenging viewing: Texhnolyze, Ergo Proxy, Kaiba, Kino's Journey (2003), Mushishi, Ghost in the Shell: Stand Alone Complex
Level 7 — "You need to read an essay to understand this": Neon Genesis Evangelion (fully analyzed), Angel's Egg (1985, dir. Mamoru Oshii), Belladonna of Sadness (1973), Night on the Galactic Railroad (1985)
Level 8 — Obscure masterpieces: The Rose of Versailles (1979), Ashita no Joe (1970), Future Boy Conan (1978, dir. Miyazaki), Dear Brother (1991)
Level 9 — Deep archival: Osamu Tezuka's experimental shorts, Toei Doga pre-Ghibli films, Belladonna of Sadness in full historical context
Level 10 — The abyss: Mind Game (2004, dir. Masaaki Yuasa), Cat Soup (2001), Midori: Shoujo Tsubaki (1992) — this one is genuinely disturbing and difficult to find for good reason.
Sources: MyAnimeList historical data, anime historian YouTube channels (Pause and Select, Super Eyepatch Wolf), academic writings on anime history
Legend of the Galactic Heroes at Level 4 is correct placement. 110 episodes of political space opera with no filler. It is the anime equivalent of reading War and Peace and equally rewarding.