I would put any kind of part-time paid work at the top of the list. Nothing much will beat a professional reference from a former employer, stating that you are honest, you turn up on time, you're reliable and you do what you're asked to do with minimum supervision. Even bar or shop work will give you that. Plus it will pay, which is crucial to many students.
ECs are fine, but they're basically just hobbies unless they can be directly related to your job. Captain of a sports team or chairman/secretary/treasurer of a society will give you leadership skills and evidence of taking responsibility. I know a few folks involved with the Student Unon who have gone into politics. Target what you do if that's why you're doing them. But generally, I'd choose ECs that you enjoy.