Well, I can refer you to some Manga... But not much of the Medieval... Full Metal Alchemist is about the closest I can come to medieval fantasy in my timeline aside from Claymore.
Demon Diary was okay, but a little naive... Summery frome Tokyopop: Gods and demons wage a neverending battle with the mortal realm as their battlefield. As with most longstanding feuds, the reasons are no longer important--hatred is a way of life. But it is foretold that one would arise who could restore harmony between gods and demons. Enter Raenef...heir to a set of demon royalty, he is hardly courtly material. The demon king assigns Eclipse to be his tutor, to mold Raenef into proper demonic shape. The two begin a journey of discovery and are soon joined by a human knight and a god-blessed priest.
Another one I liked was Chrono Crusade... I actually liked that one more than the above, but it's more Fantasy on the borderline of Religion.
Hellsing is a wonderful manga if you're into the non-sparkle(yay) vampires and the such.
Spice and Wolf is another set in near Medieval times, but not a high Fantasy. Low action and magic, but interesting to say the least.
Claymore was a great fantasy manga, I never got to read the ending though. Also in the older times, what I read of it was awesome, action packed, quiet a few turns in the story line.
One of my favorites is The main character is Mythical Detective Loki Ragnarok, Review from Mangafox: Loki, the Norse god of mischief. After causing boundless trouble in the land of the gods, Loki is banished to the world of humans and transformed into a little boy. To get home, he must collect "evil auras" (evil spirits), so he sets up Enjaku Detective Agency, specializing in strange and paranormal cases to make his search easier. For better or worse, he attracts Mayura Daidouji— a mystery-crazed teenage girl who quickly appoints herself a member of his detective agency. Soon more gods come down to the human world to kill Loki under Odin's order. Just why is Odin trying to kill him?
It's probably more cutesy that what you're looking for, but it gets deep into Norse Mythology despite that. My favorite character? Fenris. Who can't love a little dog that actually a giant wolf, that chatters on happily about mauling someone like a kid would tell about finding an interesting bug? There's a lot of magic, comedy, and serious moments. I'm a nut for Norse Mythology, so my referral to this particular book can be considered bi-est.
And D-Gray Man.
That's about all I got on Medieval or High Fantasy Graphic Novels... well, at least the ones I think you might could possibly be interested in. I usually read Fantasy/sci-fi and steam-punk or modern fantasy... I just noticed you didn't get any replies...
It's not particularly common; most fantasy series in the graphic novel department are either set in Japan (since manga is a good deal of the market) or set on modern day Earth. Still, there are a few good examples.
Conan the Barbarian, more sword and sorcery but still a classic.
El Mercenario, a Spanish comic that is right beautiful. Can be hard to find a good translation, though.
Also, there is Spawn: Godslayer, which is based on the Spawn series, but set in a medieval fantasy setting.
Though I haven't read many of them, there isn't a popular series out there that doesn't have a comic attached to it. Today, I saw some comics for Wheel of Time. I'm sure browsing your local book store will turn up something.
If you can go for manga, I would recommend:
Claymore, as mentioned above.
Record of Lodoss War, which reads like a D&D game.
Berserk, an absolute classic of the entire medium.