Dragonbox! They offer such deep, conceptual math understanding that I happily pay for them. My preschoolers play the Numbers app, and then move on to Big Numbers when they are ready. They also play around with the Algebra 5+ app, but they don't really get it until they are a bit older.
Also all the Toca Boca apps. They aren't educational in the sense of letter or number games, but I still find them very worthwhile. On their site they say that they're "a play studio that believes in the power of play. Play for the sake of play. Play without rules, levels or predetermined outcomes. Play that unleashes kids’ superpowers." They love experimenting in the Toca Lab; there is no goal other than to see what happens when you combine or manipulate different elements. They like creating forest ecosystems in Toca Nature, playing with Toca Robots, cooking in the Toca Kitchen, etc. Really, all the kids (ages 2-8) have enjoyed every Toca Boca apps that we have bought.