Kafka only wrote 3 novels, all unfinished, all excellent.
"The Trial" is the conventional starter Kafka novel. It's the only one with an ending.
"The Castle" is my favorite. A good entry if you think you already understand Kafka (you may be surprised).
"Amerika", or "The Man Who Disappeared", is an under-appreciated gem but should be read 3rd because it's the least complete and you'll need the practice appreciating unfinished novels.
Didn't he only write like 30 stories or something?
I think there might be something to an idea that people who already have considered things the way Kafka does might not value his pieces much, because other than conceptually there might not be as much there as other famous classics writers?