Simon is past his prime but getting rid of him will put the show on a FASTER tailspin. He still plays the tough judge but he's not up to it.
Louis is a fixture in the show and provides the right comic relief; although, if you have loopy Sharon in the panel with him, he can stand to be more serious.
Sharon is fun. She's playing the role Louis usually plays and it's not bad, if not totally un-constructive.
Nicole now is not as good of a fit in the panel as in her first stint. When she brings her character into being the de-facto head judge, it works; when she does it as the "pretty judge", not so much.
Gary doing the "nasty" act just sounds petty and condescending; when he bashes someone for not having a good voice, he's reminding you why Robbie became big and he didn't.
Dannii was great and always tries to be contructive but she gets petty, too. (see: Jedward, Katie Waissel)
Kelly's American-isms may not always resonate with the British public (who knew you'a doin' uptempo, owww), but she was solid.
Cheryl's method is simple but unpretentious (well, mostly). She pairs well with a strong-willed female (Dannii, compared to Rita).
Mel plays the nasty judge really well. Maybe bringing her back with the role would work, as her sass feels second-fiddle to Simon's two years ago.
Tulisa is hit-and-miss for me. She clicked with Little Mix but everything else felt off.
A nice lineup would be Louis-Mel-Kelly-Simon