I would say keirsey regarding personality types. It's understandable and very spot on for a lot of aspects of one's personality. It's also very detailed as it goes into the functions and what everything means and how it fits a type all.
I read Please Understand Me II a few weeks ago. I really did like the way in which he described the temperaments and types. I like MB too but I think that some of Keirseys observations and ideas are more accurate and more in depth.