For the shelf, hardcovers.
For general ease in reading, mass market paperbacks. They are easy to carry, fit in a purse or many pockets, and most are easy enough to read with one hand for those times when you need to eat or hang on to a subway pole.
I have a personal grudge against trade paperbacks. They are overpriced and fail, on average, to wear any better than the mass markets. I think they are a conspiracy to get readers to spend more money at the bookstore. More and more, titles available in mass market seem to tend toward genre works – science fiction, mystery, etc. “Literary fiction” tends to be entrenched in the trade paperback. The only use I have for them is for works of great length – some books are too fat to work well as a mass market.