All time best bands? That’s a tough one. I’m going to go with eras, just to help out a bit. Obviously, some could be put in different decades, depending on when you think they made their best music:
60s: Who, Hendrix, Cream, Crosby/Stills/Nash
70s: Jethro Tull, Led Zeppelin (perhaps the best band ever), Pink Floyd
80s: Duran Duran (hated them when I was young…LOVE them now), Guns and Roses, Cult, Metallica, Jane’s Addiction
90s: Ministry, NIN
2000+: There are none
Great songs that don’t come from all-time great bands (though they are good bands): Sober (Tool), Ziggy Played Guitar (David Bowie), Can’t Wash Her Away (del Amitri), Smells Like Teen Spirit (Nirvana), Paint it Black and Sympathy for the Devil (Rolling Stones), the Wizard (Uriah Heep), Anarchy in the UK (Sex Pistols), Someon Just Like You (Bang Tango), Safety Dance (Men Without Hats), Hazy Shade of Winter (Bangles), Synchronicity 2/King of Pain (the Police), most of 311’s first album, Music (downhill ever since), Little Red Riboflavin/Godzilla (Bouffant Jellyfish), Gold Dust Woman (Fleetwood Mac), Cult of Personality (Living Colour), Re-Ignition/Quickness/Rise (Bad Brains), Siva/Rhinocerous (Smashing Pumpkins)
I could go on, but I think I have already overdone it.