I was really hyped about CAs for a while, but they are sort of a mystery to me still. Even though GoL is beautiful and by far the most intriguing one, it is too hard to come across new discoveries in it. I like those which behave like machines, particularly von Neumann's 29-state and it's derivatives, attached is a document explaining how von Neumann's reproducing machine works. Another pretty one is one derived from Wireworld, it's name is something like WWEJ3 irrc. One type of such automata which I think is most interesting is actually the 2-dimensional turing machine, aka the turmite. Langton's ants are the most popular rule.