I'm super late to this, but for me the most surprising artists of 2018 were Kim Petras and Shawn Mendes! Kim was a totally new find for me but she impressed me so much with her brilliant songwriting and unique productions. On the other hand, Shawn is an artist who I wrote off as a boring teen idol, but some of the music off his latest album is brilliant. 'In My Blood' and 'Lost In Japan' especially are both brilliant!
As for the biggest disappointment, I definitely have to give the dubious honour to Ariana. I was never a stan of hers but she always had at least a couple of good standout tracks on each album, but Sweetener was just a total mess from start to finish. Even the stuff from her newer album has been disappointing imo. It's a shame because she has talent but she's definitely gone down the wrong path with this music.