My default is to say that_ Ghost In The Shell: Stand Alone Complex_ (Seasons 1&2) is the best show I've ever watched. It's my desert island show, I could watch it over and over till my eyes fall out. On top of that, the dub is one of the best in the business.
After that I drop down to my next tier, shows that have a pretty rich world that gets explored the further you dive. Spice & Wolf, Darker Than Black, GATE, and Log Horizon take the cake there, though if you broaden the descriptor enough, almost any show fills that criteria.
From there it's the guilty pleasures. Shows that are too inane to take seriously, too off the wall to normally suggest, or too laser focused on a certain thing to see much appeal. It's mostly a grab bag of shows that don't fit into the Absolute Best or High Quality monikers. Inazuma Eleven, Nerima Daikon Brothers, Ghost Hunt, Ghost Stories^, Serial Experiments Lain, Mononoke, and Ouran High School Host Club.
Do note that I've excluded anime already mentioned. I think both FMA series are in the top 20 of the past 2 decades, easily. As well, just because an anime is in that third group doesn't mean I don't love it. Renge from Ouran is so me (at least, what my friends say) that a few of them have changed what they see for my skype name to stuff like 'Renge' or 'Oh-hohoho!'. And Inazuma Eleven is a shit anime. It's bad, just awful. Some of the most off the wall and inanest dribble I've seen.... but I fucking love it. It gets me pumped and energetic and I'm glued to my seat. It's shit, and I wallow in it like a goddamn pig.
^: This only counts for the dub, which was reflavored with ad-libbed dialogue after an abysmal run in Japan.
