It's hard to pick MJ's worst song because he doesn't have many bad songs, the majority of his songs are great and timeless. But the most forgettable #1 he has is probably Ben so I'll go with that.
The list of number #1s
"I Kissed a Girl" (2008), "California Gurls" feat Snoop Dogg (2010), "Teenage Dream" (2010), "Firework" (2010), "E.T." feat Kanye West (2011), "Last Friday Night (T.G.I.F.)" (2011), "Part of Me" (2012), "Roar" (2013), "Dark Horse" feat Juicy J (2014)