Solitaire by MobilityWare is the top paid app and probably the best one at the moment anyway. Here's a link.
Yes, I know. I've never bought one. I've found Sol Free Solitaire to be good for a free one. And Card Shark Solitaire used to be free, but it's just a deluxe version now, I think. It was good as well.
My favorite one is this one. http://itunes.apple.com/us/app/id314849849?mt=8 It's a bit expensive though, but it's the best one I've played and believe me I've gone through a whole lot of them.
Hi, My personal favorite is Gamebox Solitaire. Was the first game I bought for my Touch and really happy with it. How it compares with others, I have not a clue. Just know that I enjoy it very much and it is one of my most played games. David
This sort of reasoning is why MobilityWare's solitaire remains the top selling Solitaire game and probably will through 2010, too. In my opinion, it remains incredibly popular not because it is leaps and bounds better than similar games, but because of its placement on top lists and corresponding momentum.