Answer by taking a quote from the following article:

The Sweet (also known as Sweet) is a British glam rock band that rose to worldwide fame in the 1970s. Their best known line-up consisted of lead vocalist Brian Connolly, bass player Steve Priest, guitarist Andy Scott, and drummer Mick Tucker. The group was originally called Sweetshop. The band was formed in London in 1968 and achieved their first hit, "Funny Funny", in 1971 after teaming up with songwriters Nicky Chinn and Mike Chapman and record producer Phil Wainman.

The Sweet made their UK television debut in December 1970 on a pop show called Lift Off, performing the song "Funny Funny". A management deal was signed with the aforementioned songwriting team of Nicky Chinn and Mike Chapman. Phil Wainman resumed his collaboration with Sweet, as executive producer. This management deal also included a worldwide (the U.S. excepted) record contract with RCA Records (in the United States and Canada Bell Records issued the group's music until late 1973; followed by Capitol Records).  In March 1971 RCA issued "Funny Funny", written by Chinn and Chapman, which became the group's first international hit, climbing to the Top 20 on many of the world's charts. EMI reissued their 1970 single "All You'll Ever Get from Me" (May 1971) and it again failed to chart. Their next RCA release "Co-Co" (June 1971) went to number two in the U.K. and their follow up single, "Alexander Graham Bell" (October 1971), only went to #33. These tracks still featured session musicians on the instruments with the quartet providing only the vocals.  The Sweet's first full LP album, Funny How Sweet Co-Co Can Be, was released in November 1971. A collection of the band's recent singles supplemented by some new Chinn/Chapman tunes (including "Chop Chop" and "Tom Tom Turnaround") and pop covers (such as the Lovin' Spoonful's "Daydream" and the Supremes' "Reflections"), the album, recorded at Nova Studios in London, was produced by Phil Wainman and engineered by Richard Dodd and Eric Holland. It was not a serious contender on the charts. Their albums' failure to match the success of their singles was a problem that would plague the band throughout their career.

What was the most popular song?
Funny Funny