Crobat. Not only did it never do much when it was on Brock's roster, (it was shafted since it was only a Zubat for that matter), but Crobat's in general barely get any screentime.
Geodude has had a ton of screentime, and even though Steelix was his first Pokemon...that thing has really been overused in AG. Plus they want to keep it as a Rock Gym, don't they? Leave the rock types there.
It was a pretty stupid idea to leave Ludicolo behind in the first place, it's not like it was bothering anyone on the main cast. Writers, was it really necessary to leave it behind?
Anyway, if Brock was going to pick up any of his own Pokemon he would have done it in the last new episode. It's a lost cause now, and I'd prefer it if Brock himself was given a break during the D/P saga.