I have lived through the One Direction years for 3 years now? Maybe 4 who knows. I can just tell you that I have seen them with cp 3 times in concert and a 4th time listening to them in the parking lot at Giant Stadium while waiting for cp and her friend. They put on a great show and seem to truly appreciate their audience and how far they have come.
They all sing. Niall is probably the weakest link singing wise but has improved since they formed the band. He does play guitar throughout the concerts so that is a plus in my book. They have done a good job showcasing all of them on different songs and truly keeping them a band rather than a soloist or two and backups.
Imho, Harry is the most talented vocally with Zayn second (because of his range) but the rest are talented as well. I am actually surprised he left and now, mid-tour. I always figured Harry would be the first to go, but he is the youngest so there is that factor (as you can see I know way too much about 1D but this is what the car rides to and from cheer do).
Ironically if you watch the X Factor shows you will see that Zayn is the anti-Beyonce in that he cannot dance to save his life (or he was just more inhibited than the others and had a harder time with performing).
I'm pretty embarassed I know this much about them....but if you saw cp's room and her lifesize cardboard cutout of Harry, you would understand. Fortunately for my musical tastes she has been more into Fallout Boy as of late....but she is still devastated.