How does the main character make a story interesting?

Esteemed friends interested and bored enough to read my published rants, we stand on the brink of finishing our Beowulf unit. I, having read the whole thing, have a few afterthoughts upon Beowulf as a main character.
Beowulf almost immediately struck me as a legendary unstoppable kinda guy from as soon as he wandered into the story. He's a warrior with bear-like strength who in his senior years is still able to break a dyed-iron viker sword against any surface that dare test its sharpness. With his bare hands he was able to wrestle with the cursed monster Grendel, whose skin was like iron to the blade, and tear his arm off no less. And he has no doubt of his abilities, shows no sign of fear to his enemies: his mind is unbreakable. So here's a guy with basically no weaknesses, except that he's so strong he can accidentally break his own armaments. Invincible, unbreakable, god-like: perfect. (I just know I've seen those words somewhere else...can't quite put my finger on it...some book about vampires....)
So overall I was actually disappointed in the story and in Beowulf as a character. While I was reading along I kept praying for some amount of actual struggle in the conflicts that took place. But time after time my prayers went unanswered. Beowulf seamlessly overcame every single obstacle he came across (except of course the dragon, because hey, it's a freaking dragon). Seamlessly, I say! because despite the fact he's everything but immortal, he defeats his enemies without so much as a possibility of failure present. In fact he isn't so much as scratched when facing Grendel and Grendel's mom. And yes, I know he gave a speech before he went off to kill her that was basically him stating that he might die, to me it sounded more like "Oh yeah man, sounds radical. I can probably do it without even breaking a sweat, but to be fair, if I really do die through some really stupid series of misfortunes, that's totally cool with me. Now let's blow this popsicle stand!"
I understand that Beowulf is not modern literature and to compare it with our modern expectations would be unfair. But to me it seems like one of those classics you study in high school and never speak of again until all of the sudden there's a movie about it that barely resembles the text. And in the interest of good literature I still choose to pick at its unfathomably invincible main character.
Beowulf is not charming in a way a main character should be. He's actually rather full of himself. And it's not that he couldn't compile a list of good reasons for his self-absorbedness, it just makes him a lot less interesting and memorable than a main character should be. He has the personality and description that would better fit him in the role of a minor character. Compare him to minor characters such as Emmett Cullen, that one master fencer from Seven Samurai whom I can't remember the name of, or Auron from Final Fantasy X. People like those characters and some who can't look two moves ahead demand that they be the main characters of the story, but they really work better and can get that reputation because they're not the main character. In fiction, the main character is the one who must struggle with conflict both plot-related and psychological. Pre-composed invincible characters like Beowulf and Emmett wouldn't make for interesting main characters because there is no struggle anymore, they are completely developed and would therefore make for a very boring and uneventful story (poor Stephenie Meyer managed to weave past that literary safety pylon by making her centralized character stale in those ways). If the main character is designed such that they can overcome any obstacle with minimal effort and their mind is already composed beyond breaking point without the risk of losing a shred of confidence, the story overall will struggle to be interesting. Suspense must take place. Simple rule of fiction.
Often I fight for the idea of the main character still having at least some skill or talent. If there's anything I hate, it's an unbelievably skilless main character utterly lacking in confidence and knowledge of ability to the point of contemptibility. Invincibility may kill suspense, but total powerlessness is simply absurd. As Brandon Sanderson so wittingly wrote in his review for Henroshi's Bane (a short story of mine), "I am relieved to not find myself reading the story of yet another ordinary peasant boy somehow destined to save humanity. [Henroshi] has skill, and there's no shame in it."
I designed Henroshi to be a man with a mind as sharp and ready as the blade he so masterfully wielded. This is how I see it: I give a character a sword, they must know how to use it. It is utility. The best way to challenge a character thus is to either take away their choice weapon or put them against a foe with equal or greater skill. This seems difficult at first, but hey, so is writing a novel. Also, Henroshi had to deal with a serious psychological obstacle which he had difficulty overcoming throughout the entire story. And that is how you create a main character who is unique and charismatic enough to be remembered; not for his place in the plot, but for his own skills and attributes along with the way he is developed.