Zuma's Revenge is a nice game from PopCap. It's a colorful, casual, action/puzzle title, with a simple mechanic that's easy to understand. But don't get too comfortable, as Zuma's Revenge gets difficult quickly. It's existed as a PC title for a while, and is finally ready for iOS.
Shooting little colored balls into each other is fun, and the game's great Aztec-inspired theme adds to it. It's a deep game despite the simple mechanic. You can fire balls into a gap for bonus points and figure out how to finish off the line of orbs as quickly as possible.
Zuma's Revenge HD is a no-brainer as a great game from a company with a shining reputation for games just like this. The only concern (other than the lack of Retina graphics -- at least I don't think they're in there yet, and I don't have a new iPad to check) might be the price. It's US$4.99 for a universal app. That's not actually too high for a PC game like this, PopCap's parent EA often has sales, so odds are that we'll see this a little cheaper before long. Whether you wait or not, however, you'll probably like picking this one up eventually anyway.