I love board games and definitely those you can play on the iPad. These are two board games that I have tried and find have a somewhat steep learning curve. All of these games are very good and if you have played them before you should not have any trouble with them. Just because it is taking me a while to play, I am not giving up. So if you enjoy board games and want a challenge, give these a try.
Reiner Knizia’s Tigris & Euphrates
Uwe Rosenberg’ Le Havre (The Harbor)
Zulus on the Ramparts! HD
Caylus