That is impossible for me to comprise, as it would morph and shift every few weeks/months. Some that for many years have been irreplaceable are:
TOOL I can't imagine anything anytime soon replacing these men. This quartet is the most intelligent thing going in modern music, and the only thing that makes me hold out hope that contemporary music is not completely lost. As far as I'm concerned, the greatest band since Zeppelin, and possibly the greatest band of all time. The places they can take the mind are simply unparalleled by anyone I've ever heard, to include Pink Floyd, the Beatles, Zeppelin, etc. all of whom I appreciate greatly. And no drugs required

This band has literally affected, even changed my life. Beautiful, tragic, cathartic, transcendental. Gods of their craft.
Rage Against The Machine It is unsure whether this quartet is still alive, as they have reunited and are currently touring, though have no plans of producing new music. All of their tracks are just as politically alive and viable today as they were 10 years ago, and much of it is so historically and politically intertwined that it often goes over my head. At first I was turned off by the hip-hop infusion, and the harshness of Zach de la Rocha's voice, but I've come to embrace it as one of the greatest catharses in music I've as yet discovered. And everyone knows Morello is THE MAN on the G-tar.
Nine Inch Nails Trent Reznor, as the creator and only consistent "member" of this band, is absolutely spellbinding on anything with keys. He can tickle ivories with some of his best fellow contemporaries, but in a strangely macabre and melancholy manner that I have yet to hear reproduced. His emotion is extremely raw and always laid bare for his listeners, and from vicious, angry tirades, to political diatribes, to morose dirges, he never fails to inspire, amaze, and sometimes disconcert. I think what I find most laudable of his musical traits is his competence with beat production. He synthesizes beats/rhythms that make modern hip hop artists look silly, and does it without pandering to whatever contemporary pop-culture sentiment is "in" at the moment. Finally, he is singlehandedly revolutionizing the music industry with his pro-pirating coercion of record labels into releasing their death-grip on artists' talents, and helping force amendments of various other corporate "ailments" from which they so often suffer.
Nirvana Raw, emotional, creative, revolutionary. These three are gone but not forgotten. Songs like "You Know You're Right" are so capable of ripping my heart out that I can't help but wail along with them (when alone in my car). I doubt anyone here needs an explanation for why they are noteworthy. Perhaps overrated, but this never seems to slow my enjoyment of them. Due to the lack of new material, they're on slow rotation, but they always find their way back on every once and again.
Jack Johnson Beautifully poetic, an excellent and enjoyable guitarist (quite the practitioner of barre chords), simplistic, and relaxing, Johnson covers the gamut of song subjects and yet somehow is dismissed so often as a romantic beach bum. His songs range from subjects like love, simplified living, depression, politics, world order, and...of course...surfing. And that is in no way a summary. His are songs that demand to be sung along with, yet are far more intelligent than the standard radio fare of today. Additionally, his Solar Powered Plastic Plant in which he records is setting an example to other musical celebrities that "going green" should be just a fad and a motto.
Ben Harper A very intelligent and VERY talented musician who plays multiple instruments, and plays them well. His style is uniquely varied, and like many artist's subject matter, so varies his style. From blues, to lullaby, to flat out rock, to beach-going tranquility, Ben is an amazing artist with clever lyrics and amazing music. His popularity continues to grow, and rightly so. He regularly collaborates with other great artists like Jack Johnson, Xavier Rudd, etc. keeping his music fresh, and the flow of ideas that produce it just as novel. Just good listening.
Bob Marley Please. Do I need to explain?
Sublime Unique, innovative, and characteristic of the Beach mentality and lifestyle, this trio, in a very short career, immortalized not only their anomalous musical style, but their renegade philosophies, and their peace, love, and party attitude. How anyone can not appreciate this band is beyond me. They're fun to listen to, can usually brighten your spirits on a miserable day, and are practically incomparable for good party music. Virtually everyone can get behind "What I Got" by both singing the lyrics, and trying hard to walk the walk. And what guy in this forum doesn't crack up at the lyrics of "Wrong Way" or "Date Rape"??? Come on... Musically talented, good times, good memories had by all.
Metallica Should require no explanation.
Led Zeppelin Should require NO explanation. In contention for the title of the greatest band of all time.
A Perfect Circle To end where we began (actually on number 11), Maynard James Keenan's (singer of Tool) side project, currently on permanent hiatus, provides the estrogenical balance to Tool's testosterone driven onslaught. Whereas Tool is claimed by Keenan to be the catharsis for his masculine side, APC fits the bill for his internal feminine. Tool to be sure, has a level of sentimentality above and beyond most of their contemporaries, however A Perfect Circle has the opposite, which is more estrogen, and a lower balance of testosterone. Together, they compliment quite well, and though given the choice, I'd take Tool as my one and only, APC wouldn't be too far behind. Again, contemplative, cathartic, smart, and tragically beautiful.
VERY notable mentions which would likely be interchangeable with some of these, would be Red Hot Chili Peppers, Death Cab For Cutie, Bon Iver, Jewel, Eddie Vedder, DEFINITELY ADIEMUS, Elliot Smith, Flogging Molly, and probably a few others.
NOW, all that said, I often just prefer soundtracks, or classical music, sans lyrics altogether. Hans Zimmer, James Horner, James Newton Howard, Ennio Morricone, Danny Elfman, etc. are often gracing my speakers more frequently than most bands. Aaron Copeland, Mozart, Bach, Chopin...they aren't unheard of in my repertoire, though certainly see less play time than the aforementioned.
Some great soundtracks in no particular order include Legends of the Fall, Cast Away, Fight Club, BLOOD DIAMOND (love that one), Avatar, Apocalypto, Shawshank Redemption, etc.
I'm sure that's enough to lose most of you, so I'll give up now unless anything else comes to mind.
