I can't narrow it down to one, so I'll list three as my three most readable.
Mark McElroy's Bright Idea Deck was the first Tarot deck I bought, and it was designed to be read right out of the box, even for the Tarot neophyte. Every card has pictures of people -- contemporary people often dressed in business attire -- doing things. The pictures are line drawings with bright colors, so I can always tell what's going on. There are keywords, which I generally ignore.
When I saw the first images from Patrick Valenza's Deviant Moon Tarot, I knew I had to have that deck. I connected with the deck immediately and it has not let me down. Now I use the borderless edition. The figures have kind of a surreal, cubist cartoony look, which are expressive in a way that realistically proportioned figures cannot be.
When I saw the Alexander Daniloff Tarot, I responded to it similarly to the Deviant Moon. It seems a fusion of modern cartoon art and the sacred art of the Renaissance.
So, what do these have in common that makes them especially readable (for me)? The cartoon art makes them all easy to see without straining my eyes. I always know exactly what the depicted figures are doing. Which brings me to the second characteristic, that the cards have scenes of people doing things. Each card could be the illustration to a story. Finally, each deck shows a variety of different activities. In the Deviant Moon and Daniloff Tarots, they are closely derived from the RWS system. In the Bright Idea Deck, the relationship between that and other Tarot systems is less apparent, but it is still there.