If I had to rank my favorite X-writers it would probably be something like: Lobdell, Claremont/Whedon in second, Kelly, Casey, Gillen, Fraction. I know Fraction is almost the death kiss for most people but he was doing something different with the X-Men, and while it wasn't perfect, I liked what he did from forming Scott's Alpha team, the Utopia crossover, to Second Coming. Gillen, Casey, and Kelly are all writers I really feel got short-changed on their runs. I wasn't reading comics during their times, but after hunting down back issues Kelly and Casey's runs are bright spots in my collection. I will always remember Kelly is the writer that made me fall in love with Marrow. And Casey's all male team, drawn by Sean Phillips, was something I went in expecting to hate and left wishing he'd kept writing through til the Reloaded relaunch. And Gillen really should have kept on with his plans after AvX. Not that Bendis is doing a poor job. I just loved how he wrote these characters.
And I'm actually a little shocked at the Whedon hate. I mean, to each his own, but to me he got the X-Men back down to basics and wrote and epic 25-issue story that can be reread as if it were its own graphic novel. It's the only series I've re-bought 3 times- single issues, digital, omnibus, plus I've been considering buying the two trade volumes!