If the Beatles were the best band of all time, who was second best?

Well have to be honest here, like some other answers mentioned, the Beatles were incomparable in the influence, impact, quality of songs, popularity etc they had in such a short amount of time (1963–1969). No other band really comes close to this, even during 40+ year careers. The Beatles are the best selling band in pop history – 800 million records sold. There’s no other band or solo act that comes close to that.

List of highest-certified music artists in the United States – Wikipedia

List of best-selling music artists – Wikipedia

Although Elvis Presley comes the closest – his claimed sales numbers are dubious in comparison to the Beatles considering the certified sales numbers are about 50 million less. I would guess that Elvis and Michael Jackson have both sold around 300–400 million-ish records, which would still put them quite behind the Beatles.

Other bands like Garth Brooks, AC/DC, Rolling Stones, Queen, Led Zeppelin, Beach Boys, Pink Floyd, Madonna, Elton John, Celine Dion etc are all behind Beatles/Elvis/MJ.

Yes I know popularity doesn’t necessarily equal the best, but how else does someone measure how ‘good’ or ‘successful’ a band is if not by sales? By sales, the Beatles is objectively the best band, and its unlikely that any other band or solo artist will surpass them.