The Verge has done the best all-around job of reporting on this topic, IMO. Check out
The Best Tablet You Can Buy. Having used all of these, I completely concur with their findings. It's hard to beat the iPad Air, but for someone who wants something a bit smaller, then the iPad Mini is the way. If you must have an Android tablet, the Nexus 7 is the best experience, with the least stuff in the way, at a great price. Finally, for the Windows die-hards who want a tablet form factor and run all the old apps, Surface 3 is the top choice.