Urgh, this is hard for me, but probably vocals. It's the reason I first started listening to music, when I first heard Carrie Underwood's voice summoning me out of the darkness.
But also songwriting is great. It's funny that my two faves are polar opposites. Carrie's the best vocalist of the 2000s, but her songwriting is TERRIBLE, while Taylor's songwriting is incredible, while her vocals are cringeworthy.
Songwriting is the key that underpins everything. That is why some great singers flop because they lack hit songs.
American Idol and the Voice show the importance of great songs. Almost all the acts on those shows who have proven they have decent singing abilities subsequently fail to be successful, and that is down to lacking hit songs.
Not every songwriter has star power though, so this enables singers who do have it, use songs written by others.
Pop music as a genre often makes use of professional songwriters. In Rock genre, it is generally expected that the artist/band writes their own music rather than use songs written by others.
