The toolbox systems- GURPS, HERO and M&M:W&W- tend towards being the most rules intensive.
1) M&M:W&W (Mutants & Masterminds: Warrirors & Warlocks): is the fantasy supplement for the D20-derived supers game, Mutants & Masterminds 2Ed. As such, it is as complex as any of the D20 games. The advantage over using W&W over D&D is- despite their similarities, W&W is much more flexible because it is a classless, point-based system with an open structure...like most toolbox games. This is my 2nd favorite toolbox RPG. If you go for it, the Books of Magic supplement will probably also be helpful.
2) HERO (aka Champions): my favorite RPG, bar none. Because the default setting is supers, you need to tone it down to run low fantasy. Fortunately, that's pretty easily done- mostly, you just set a low points ceiling and maximum power. Buying the sourcebooks like Fantasy HERO or Ultimate Martial Artist is less about necessity and more about standardization and lightening of your workload. IOW, convenience. Besides the flexibility, my favorite aspect of this game is that, once you get through character creation- which can take hours because of all the options...and math- you almost never need to use the books at the table. Most of what you need is on your character sheet. That means that, unlike some other complex RPGs, play at the table can be fairly rapid indeed. Point-based, open structure.
3) GURPS: probably the grittiest of the bunch, and its default rules tend towards low fantasy. Lots of sourcebooks covering RW cultures, weapons and martial arts styles. (Like many gamers, the sourcebooks are my favorite GURPS products.) there are supplements that let you expand into high-fantasy type campaigns. Point.-based, open structure. It is one of my least favorite systems, but it can't deny its power, flexibility, and the fact that it is probably closest to your goal right out of the box.