I use an app called wordfeud. I think I paid 4$ once, and that's it. I've played thousands of games for that 4$. It is not going to win any design awards, but it is good enough. More importantly, it is dependable and just works. I've soundly beaten some beginners and have been beaten beyond recognition by experts. It has tons of users - from beginners to very, very strong players and everyone in between.

Not sure who owns wordfeud, hopefully not EA, it would break my heart :(