Like the title says, list bands and artists that you feel are irreplaceable. Mine are:
Rage Against The Machine - Probably the guys who birthed Nu Metal and gave new life to Alternative Metal. I have not yet heard a band that mixes hip hop, funk, metal, and political views like these guys. Zack De La Rocha's piercing vocals, Tom's unusual guitar playing really make them stand out when trying to compare them to every other band in their genre. No one sounds like them.
Red Hot Chili Peppers - The only remaining members since it's birth are Flea and Anthony I believe, but they're really the soul of this band. The mixture of funk and rock is just candy to my ears. I've heard other bands who fuse the two genres together, but really I can't say I've ever confused a band with them and consider them to be irreplaceable.
Carlos Santana - I really don't know what it is about this guy that makes me feel he has no substitute. I have heard other guys imitate his guitar tune and play style, but it could be his way of adding latin guitar to a mixture of genres, or maybe he truly doesn't have a substitute. He's been featured in alternative metal, hip hop, R&B, pop, world, and a few other genres and you just always know it's him when you hear him and if you find that it's one of his imitators, it just leaves you feeling disappointed (if you're a fan).
Snoop Dogg & Dr. Dre - I don't know about Snoops reggae stuff or both of their later hip hop music, but when they were a duo back in the day, they have music that still gets played constantly to this day.
Tupac & Biggie - Pretty much the same as above.
P.O.D. - These guys are almost like Rage Against The Machine while at the same time being nothing like Rage Against The Machine. They also mix hip hop, funk, and metal just like RATM, but also include latin rock and reggae in their later albums. Sonny's vocals don't compare to Zack De La Rocha but I haven't heard anyone else sing like him either. Marco doesn't play anything like Tom Morello but he mixes up his style a lot which is cool. They're a religion heavy band but a lot of their tracks also appeal to non believers. They're most notable song which had a lot of success in sports and commercials was "Boom" which actually started to annoy the hell out of me lol. I didn't think it was a bad song but to hear it every time I tuned into ESPN started to get a little too repetitive.
Matisyahu - This one has no substitute for a reason. He heavily puts his Jewish beliefs into his music and has a unique voice. I also like that he has unique singing where he's not saying any words but is just singing along to the music. He just sounds so passionate when he does it that it makes people sing along with him. I love that he fuses reggae, hip hop, and rock into his music and has his own rapping style. He is probably the most unique artist I have on my playlist.
Don't hate that most of those artists were once mainstreem lol. Anyway, those are just a few artists I don't find to be replaceable. What are yours?