In the last 10 years here are the top 5...
Fraction
Gillen
Lemire
Aaron
Bendis
Be the change you wish to see in the world.
When I read the title of the thread, I immediately thought about Jeff Lemire's run. His time on Extraordinary was so forgettable. Just filled with stupid ideas poorly executed.
Jason Aaron was also pretty bad on Wolverine and the X-Men.
I also hated Spiderman and the X-Men so Kalan gets no love from me either.
Humphries X-Force was also bad.
Bendis for me basically broke the xmen. After Avx I was hoping for some interesting conflict between the two teams. Instead we got this God awful drama about what should be done with young o5. I wasted so much money
I remember reading an "Wizard" article back in 1999 after Seagle and Kelly quit about how hard it was to keep X-men writers. Bob Harras must have been a real hard ass! I remember reading that in 1996, Mark Waid quit writing X-Men after only 6 issues...
Thinking about my choices again, I can't say that Austen was the worst (that probably goes to Lemire followed closely by Aaron), as there were actually a few things I remember liking about his run.
I liked his use of Juggernaut (after Joe Q wouldn't let him bring back Colossus), Polaris (with her craziness being due to extreme PTSD from surviving Genosha), Northstar, Stacy X (he did write some of her, right?) and Sammy Pare (that's right, I said it): the thing that made him so bad were his plots, mainly the Draco, the Church of Humanity and the blatant Romeo and Juliet rip he did. Woof.
When it comes to Lemire and Aaron, I honestly can't think of a single redeeming factor for either run. Truly horrid.
What can I say but, "I love comics."
Austen ruined Stacy X, actually. Casey built up a complex character and Austen turned her into a mess in love with Warren. Then he removed her to make way for the Paige-Warren couple.