It is hard, because I love music. I'll share a few that I listened to on repeat years ago when I first heard of them:
1.
Yoshiwara Higanbana Ending theme. I love the opening theme too, but someone mentioned it already.
2. Chou no Doku Hana no Kusari
Opening and
Ending Theme
3. Jooubachi no Oubou Menou's
Opening and Kaguya's
Opening Theme
4. Tsumikui ~Sen no Noroi, Sen no Inori~
Opening Theme
5. Sachi no Tenbin
Opening Theme
6. Monochrome Wizard ~Kuro no Sanka, Shiro no Chinkonka~
Opening Theme
7. Kankinkon ~Sentaku no Hibi~ Hikaru's Route's
Theme
8. Hakuouki anime Kitan's
Opening, Hekketsuroku
Opening, Sekkaroku's
Opening, and Reimeiroku's
Opening. The ending themes were just as good too
Kitan's ED,
Hekketsuroku's ED, Sekkaroku's ED
Okita,
Hajime,
Sanosuke,
Shinpachi, and
Hijikata. Reimeiroku's
Ending theme. Hakuouki has a lot of good OP and ED, especially the OVAs. I want to share the theme from the movies too,
Movie 1 and
Movie 2. Movie one's theme song has very beautiful lyrics.
I'm sure the Hakuoki games have just as beautiful Opening and Ending theme, but I never got around to the game to really listen to the songs. I watched the anime only even though I have the games on the shelf.