I've been using this Eheim for the past five years. It's been rock solid (except for when I knock it into the tank LOL).
I feed pellets and It has never clumped up on me. It has a feature where it pumps a bit of air in to help keep it dry I think. The opening sizing is nice because it is a sliding door so it's virtually infinite how much food you may want to dump in. You can also do handy things like double drops (the hopper spins twice).
This one.