Grindstone is the best (and on Arcade (no in-app purchases)) - I finished all the levels long ago (but they my have added new ones) — just the daily competition (The Daily Grind) is a lot of mileage for a long time player. I spend easily an hour a day grinding - it’s the best!
Letterpress, since the launch of the iPhone, is still the best word game around if you ask me.
So simple, and yet so complex. A little bit of stratego in terms of holding real estate, but a vocabulary challenge at it’s core.