Until November 1, 2018 Ultra Rare cards were the most powerful category of cards in the game as well as the rarest, both in obtainability as well as variety. They are signified by five stars in the lower left hand corner.
Ultra Rares in general have high base stats compared with Rare and Common cards, and very high maximum stats. As well, they usually possess a fairly high rate of increase in stats. Their regular versions have a maximum level of 60, and their EX versions have a maximum level of 70. Because of this, they posses a very high stat potential. They are the most sought after group of cards in the game for this reason.
The drop rate of Ultra Rares in the Legends Card Pack is very small, making them very difficult to obtain them in this way.
Special promotional card packs are periodically released, and usually possess above normal drop rates of Ultra Rares, though these packs usually cost real money in order to draw from them. Often times, it can cost up quite a bit of money simply for a somewhat realistic chance at drawing one.
Ultra Rares can also be acquired through trade, although the price is usually quite steep. They can often be found with prices in the triple digits for even the cheapest ones. It should be noted that some "fake" Ultra Rares can be found for quite cheap, but their stats are often below average for cards of their category.