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
Mind Game at Level 10 is appropriate. Masaaki Yuasa directed it like he was having a fever dream. It is genius and completely unwatchable by conventional standards. I loved every second.