Good for You by Selena Gomez. I am still honestly shocked that it's been in the sales top 10 ever since it's release.
Jealous by Nick Jonas. After Chains flopped (it was actually the first single, but got re-released after Jealous's success) I didn't think that anyone would care about a second Nick Jonas single, but radio embraced it immediately and essentially resurrected his career
Rude by Magic! I heard the song in late 2013, a pop station was playing like "on the verge artists" or whatever and I thought "lmao this song is so bad nobody is gonna wanna hear this".... 7 months later it became a multiple week #1
Shower by Becky G. It peaked at like 16, which I never thought it would, I predicted like 36-40 range. A lot of 2014 releases surprised me tbh.
FourFiveSeconds by Rihanna. It obviously didn't smash as hard as her other lead singles, but I thought it wouldn't even see the light of day in the Top 10. Proved me wrong by peaking at #4.
Bad Blood by Taylor. I thought that this was gonna be her first real flop single, it's the worst song on the album imo. However, the remix and the rent-a-rap feature came out of nowhere and with the massive hype and celeb cameos the video garnered, it went straight to #1. Still don't think it deserved it though, it's easily one of her most uninspired and bland songs.