Way of the Samurai 1, 2, 3, and 4.
Especially 4. In way of the Samurai 4 not only do you partake in sword combat in that, like it's predecessors, can kill a person, and you in one hit on it's highest difficulty, you can collect, disassemble, and reassemble swords and do some customization of the benefits the sword gives. The customization is largely luck based though. You can improve your swords, and getting deep into it requires some effort to get the materials.
You can get quite a variety of swords from assorted katana styes,to foreign swords, to bamboo swords, to large, menacing cleaver like swords. Also the ninja-to and wakizashi, and nodachi styled blades are in the game, some more menacing than others.
Not just swords either! Spears, hand claws, Jitte (think of a sai cut in half), rifles, pistols, even a kitchen knife and so forth are also in the game.
As if the mind boggling possibilities in sword appearances weren't enough, there's easily over a dozen combat styles to master! Spear, unarmed, and sword combat, especially. Dual Wielded swords are a possiblity as well.
You can even take up the chance to -build- your own weapon style.
Enemies vary in power. There's even the option to let other people's characters appear in your game as duelists to fight! If you win, you get to claim the sword they carry, and one never knows just what you'll get! Will it be a crap sword, or will it be some maxed out monster thy painstakingly created?
And if you ever get tired of fighting yourself, there's almost always the possibility of stumbling across a duelist fighting other people! Yeah, you can find swordfights to watch!
And that's just the combat related stuff. The story is full of twists, and turns as you deal with the people in town.
The customization is wonderful as you earn samurai points to unlock more rewards with every playthrough, and your swords, and items can carry over. the ability to unlock, and buy in game, and through DLC a decent assortment of styles is pretty awesome.
Accessories can be resized, rotated, and placed prettymuch whereever on your character creating some strange appearances like a samurai man in a thong wearing a giant basket over his torso and a devil mask on the basket.
Also you can earn unlocks to play as women, though it's just an aesthetic choice.
It's definitely a gem that flies under the radar of a lot of people.
Way of the Samurai 3 is pretty nice as is 2, and 1, but as you play the diffirent installments, you can see something of the evolution of the game.
Edit:
If you're willing to go really old school, Bushido Blade 1 and 2 is one of the most unique sword fighting games -ever- with limb damage, or even instant death if you are hit in a vital area. There's a wide variety of weapons to pick from in that game, too.
Sword combat styles vary pretty decently.