Since I don't know how to put video links, I'll just list and comment.
FF13, Saber's Edge: Sounds like there is a real tension in the air. Things just got really urgent when this started playing!
Pokemon Battle Revolution, Mysterial's Theme: You've fought through a ton of difficult opponents to face him, and he's no pushover! This song conveys the ultimate challenge quite nicely.
Super Mario Galaxy 2, Final Bowser Fight: Probably the most epic song from any Mario game. Ever.
Mario and Luigi Bowser's Inside Story, Final Boss: Surprisingly melancholic for a Mario game. Also incredibly awesome.