I would say endos, bunnyhops, and 180 rollbacks are several great skills to have.
There is no 'should' in a sport called freestyle. There are natural steps to progression in tricks, but there are so many variations to tricks that you really can just learn the one trick and nothing else, then that will brach sideways into tricks that you didn't expect to learn. I know a guy who could do hitchhikers but couldn't do almost any other flatland trick. He's got a lot more tricks now, but he doesn't have a lot of the 'basics' that he could have.
Just go to www.bmxtrix.com
-> how-to -> flatland or street/dirt/vert and pick what looks fun. Keep in mind MOST of the easier stuff is towards the top of the list. But, pick what is fun and don't worry about 'should'.