Tough "Best" can mean a bunch of things. People buy iPhones and always think the newest one is the "best" phone out there, but most people buy them because of of the Easy User Interface.
With That said.
4000/5000 feature wise are the richest, but most difficult to grasp because of all the options. Best Sound Quailty and swing, 60/3000 wins. Cheaper, but still great options 2XL, 1000 w/ JJOS and new pads. If you want a newer MPC, I suggest 2500
is a great medium, besides the 60, its my favorite. Honestly though, just look for deals out there, all MPCs are awesome IMO