Well, nothing does D&D better than D&D. But I like the d20 Modern character classes, too, and there's no reason you couldn't run a medieval fantasy game with the d20 Modern rules as your engine, especially if you're going for more of a low-magic feel.
Low-magic [or hidden magic] historical games, in particular, would work nicely --- Skull and Bones, for example. I think the occupation + base class system allows a lot more flexibility in designing characters than D&D.
Incantations work nicely with the historical / hidden magic mileiu, too, although getting a few fire-and-forget spells at 4th level or so [when you can take your first level of a spellcasting class] can also work
And action points are da bomb.
Many D&D core and prestige classes could be adapted as d20 Modern advanced or prestige classes, if you want to go that route.
You might need to tone down Agile Riposte and Melee Smash, among other things.