My own app will soon function as a simple AAC app, but if you're looking for a full-featured app, GoTalk Now from Attainment Company is popular and reasonably priced. They even have a free version!
I'm partial to GoTalk Now because it's the only AAC app that supports the Switchamajig Controller. Just like you define buttons to speak, you can program them to activate switch-adapted toys and other equipment. Their support is only experimental right now, which means they reserve the right to change the way it works, but it's nice to have everything in one app.
Then when you activate the panel, you can speak and control equipment on the same screen.
Give it a try and tell me what you think. I have some ideas for ways Attainment can improve the support, but input from people using it is much more valuable than either their or my guess about what you want.
And I'm willing to support anyone who wants to add the same support to their AAC app. If you're a developer, email me. If you have a favorite app and want it Switchamajigged, point the people who wrote it at this post and tell them to get cracking!