There are a lot of Pokemon cards that are considered to be rare, but some of them are really expensive. Which ones you should buy depends on what your goal exactly is.
If you're not worried about money or budget, go for Shaymin EX. It was used by players who competed in competitive games and by amateurs who played because of its ability to Set Up, which allows players to insert up to six cards from their deck when a Pokémon is put into play.
It is extremely powerful and can turn around even the most tightly contested games. They are highly sought after, however, and are able to fetch very high prices on auction sites. Many players value their possessions of these more than other packs.