I'm not a solitaire fan, but I think that a computing platform is not complete until it has a solid solitaire app. Especially the multitouch iPad. Solitaire Classics HD is the perfect solitaire for iPad. It looks and plays beautifully.
That's what really makes this game: the really pretty graphics coupled with the way you play it, throwing cards to their positions with a simple finger gesture. It feels as close as real cards as the iPad could feel.
Solitaire Classics HD includes normal and hard versions of Klondike and Spider, as well as Freecell and a secret bonus game. There may be other solitaire apps with more kinds of games, but unless you are specifically looking for those games, this is the one you should get. [iTunes]