Her Majesty's Dog is the perfect example. She is a powerful sorceress who has a demon weredog bodyguard. He can't help obeying her--even if he doesn't want to.
Another example is Shounen Dolls. She can literally undo their existence if they don't do what she tells them to.
Also Barajou no Kiss

This is the key point. I don't want devoted vassals who would be completely obedient no matter what. The girl should have the actual power, or the guy must be bound by a spell or something like that. It doesn't have to start out like that, it can happen halfway through the manga, for example.

Captive Heart a curse not a spell, though it's not really obeying... it's kinda complicated...
hmm Watashi ni xx Shinasai! perhaps? blackmailing, but then turns out to him liking her and love triangle etc. - the usual stuff

________________
in need of romance?
uncommon, sad, sweet

Kamisama Kiss perfect for what you want
and of course InuYasha,
who doesn't know this manga/anime? xD

________________
lιvιɴɢ ιɴ α yαoι woɴderlαɴd
-QaF-

Ooku

________________

Kamisama Hajimemashita kind of? He does have to obey her since he's her familiar, but she doesn't usually order him so it's not a major part in the manga.

Haha, This is one of my favorite types of romance manga xD I definitely know a few.

Blood+ Definitely. Saya (Protag) commands her Chevalier, but when he loves her sometimes it's difficult...He must disobey. Saya is such a badass though. I recommend it to the max!

Inuyasha Eeh, You've probably heard of this one already though xD Just thought I'd recommend it just in case :>

Can't remember some, I might come back if I find some more.

Mugen Spiral, though it was axed and you might have a problem with the ending. It's particularly funny and the female lead is basically... awesome.

________________
Shoujo is a waste of time that makes me happy.