I am looking for a romance manga that has strong female MC. Longer manga is preferred, but I am open to any suggestions.

Akatsuki no Yona - this manga is up to chapter 153 and it’s ongoing. The main character Yona is very strong and doesn’t take anyone’s shit.

Faster than a Kiss - this manga is completed and about 50 chapters long. The main character is definetly strong and doesn’t take anyone’s shit. But, it’s a teacher x student thing, just warning you. It’s still just a cute Shoujo manga though, and teacher x student isnt my thing, but I gave this one a try and it’s actually one of my favorite manga now.

Akagami no Shirayukihime - ongoing and on chapter 93 now. The female lead is quite strong. It’s similar to Akatsuki no Yona.

Others:

These aren’t really manga but webtoons. They all are drawn in the same manga art style tho, and with an added bonus they’re all fully in color. Unlike manga, they are read left to right. You can usually find them on manga websites or http://webtoons.com.

Cheese in the Trap - completed but still being translated and super long. I think this main character is one of the strongest I’ve seen. She’s one of my favorite female leads of all time and this webtoon is a master piece. I highly recommend it.

Our Relationship Is... - completed. Great female lead.

Winter Woods - completed. More realistic style of art but the art style was still beautiful and I didn’t have a problem with it. Of course, the female was great.

My go to for this type of thing is Yandere Kanojo, it's a 4-koma with a surprising amount of story and character development. (note, you need to read both the magazine and online versions. They're both important.)
Also up for consideration is it's alternate universe version, Onidere.
Dengeki Daisy is also good, though I don't remember how much shit she takes. She's pretty strong, even though she is not a fighter and kind of out of her depth.
Angel Densetsu has a pretty great female lead, but she's just one of the leads, and only shows up a volume or two in.
All of these are completed, BTW.

Gokusen Yankumi takes no shit. It's long, it's hilarious, it's basically perfect.

Some of my other favourites are:
Oresama Sensei No overt romance yet though.

Chang ge xing This manga is epic in scale and gorgeous in execution. However the romance sometimes takes a backseat to the story. Though it is a very important part of the plot.

Legend of Maian Dropped though 😔 😔

Superior and its sequel

Vampire Juujikai

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