Matt has the best vocals on the show by a countrymile. The range of notes he can hit is astounding and the way he can wrap a song to make it his own is extremely admirable. There is also a certain star quality about Rebecca that actually is quite reminiscent of Leona Lewis. She has a very classy and faultless voice.
Mary sang well on the live finals but I don't quite like her. It just doesn't seem what the show is about. She has a very powerful voice and because of that she can sing a lot of those big songs but let's face it, she's not going to be "the next big recording artist." Even if she wins, she's simply going to be another Susan Boyle. People are amazed because she is fat, old, and ugly but despite her bland lifestyle, her voice is surprisingly good. She's just going to be a television phenomenon for 2 months then nobody is going to care anymore.
Aiden is actually quite good but I just don't like him that much.