From a purely personality point of view, I'd replace Bruno because I get sick of his antics. As far as I'm aware, he only exists as a tv dance judge with no current activities outside of that.
Any of the former professionals on the show could replace him really, though please God don't let it be James Jordan.
I know Len has a Dance Academy but I have no idea how much input he has in between his various tv commitments, but he's the only judge with a true ballroom background.
I could see him being replaced someday by Anton, though I'd hate it if Anton used the opportunity to gurn and quip in an attempt to build up some imagined future tv career.
Darcey is an absolute gem and I love the way she is always encouraging and any criticism she offers is constructive and goes alongside genuine tips and ideas on how the celebs could improve. I'd hate to see her go, but if she did, Karen Harding would be a great replacement.
But of all the judges, Craig is the most relevant as he still earns part of his living as a choreographer, so it's a shame he's been cast in the way he has, leaving him little room expand his function, but he's definitely the best candidate for Head Judge.