I'm only listing a few that I feel we are sorely missing. I expect you to have them all up in the next 24 hours! Hop to it Anime Curator In all seriousness though, I really just picked a few anime from each group that I thought most people might have seen (and that we are missing).
Magical Girl series
Cardcaptor Sakura (The original series of 70 episodes)
Cardcaptor Sakura Clear Card series
Selector Infected WIXOSS
Selector Spread WIXOSS
Lostorage Incited WIXOSS - I believe some of our members have some strong opinions on the Lostorage series
Lostorage Conflated WIXOSS
Space Opera
Bodacious Space Pirates
Bodacious Space Pirates Abyss of Hyperspace
Knights of Sidonia Season One
Knights of Sidonia Season Two
Galaxy Railways
Galaxy Railways A Letter from the Abandoned Planet
Romance
Engaged to the Unidentified
Isekai
Log Horizon Season One
Log Horizon Season Two
Overlord (Season One) - somehow we have Overlord III but not I or II
Overlord II
Scifi
Solty Rei - honestly, it is partially a Space Opera too..
Harem
Kanon
And last but NOT least, Reverse Harems
Fruits Basket
Hiiro no Kakera ~ The Tamayori Princess Saga Season One
Hiiro no Kakera ~ The Tamayori Princess Saga Season Two
Uta no Prince-sama Maji Love 1000 (season one)
Uta no Prince-sama Maji Love 2000
Uta no Prince-sama Maji Love Revolutions
Uta no Prince-sama Maji Love Legend Star