Super Mario RPG
This is the first entry in the Top 50 from the "Final Battle" category. Final Battle music should kick ass, of course. After all, this is the specific thing you've been pouring all those hours of gameplay into; this is where you save the world, once and for all.
Super Mario RPG is one of the easiest games ever produced, and its final battle doesn't disappoint; you would have to be so woefully unprepared to get killed by Smithy that you almost would have to have done it on purpose. (You got this far, after all, so unless you've been intentionally trying to play a low level game, your level won't be a problem.) Even without the invincibility items (Lazy Shell, Super Suit), you're not getting killed unless you're not paying attention at all.
This, I should mention in case you don't know, actually isn't the FINAL final battle music; it's the tune for the first of the two back-to-back Smithy battles, and the better of the two. Very appropriate for the weapons factory background, whereas the second and final phase is more of a Rule of Cool thing where you fight him deep underground because the fiery background is awesome.
By the way, Super Mario RPG is close to unique in that you never see the Big Bad even once until the final confrontation. (In fact, the first time I played the game I assumed, right up until the Exor fight, that Smithy was the big freaking sword stuck in Bowser's Keep.) There are two other RPGs I've played that are like that: EarthBound, which makes a very big deal about how threatening Giygas is and then follows through with a final battle sequence that will make you wet your pants; and Final Fantasy IV, in which the guy you spend the whole game THINKING is the Big Bad turns out to be a good guy that was under mind control the whole time. (It works better than it sounds.) And don't yell at me about spoilers, either. The game's probably older than you are, for Leviathan's sake.
In most RPGs the Big Bad haunts your steps at every turn, and you usually confront him a few times before the end. (Even Final Fantasy IV kind of falls in this category.) Not so in Super Mario RPG and EarthBound, and they both pull off the big reveal well, albeit in completely different ways.
This music is also the second runner-up in "Best Video Game Music to Use For a Boxer's Intro Music."