well, my 2 cents..
there is no better mpc because almost every iteration has its strong points and its weaknesses but also it really depends on your needs.
i started with an mpc 2kxl (but i already had the 950), i needed a strong sequencer especially for live use, the 2kxl i soon found out was good but too limited for my needs, so i hunted for an mpc 4k , and i found one (this was about 2007 i think) fully expanded for very reasonable price. i was foolish enough to bring it live on stage and travel with it , inside a big a$$ samsonite suitcase

i loved every bit of it, especially the keygroups , they are so advanced the new mpcs dont even touch them. great synthesis capabilities, and of course a great sound. but its a machine built for the studio, it soon developed issues so it was on repair very often and this became so frustrating i ended up selling it. of course i regret it. then i tried both the 1k and 2.5k and when i put jjos2xl on those, wow, i was flying. the sound is not good as on the 4k but they have their punch imho, the sound design is not deep as on the 4k but at least you can do keygroups for melodic instruments etc... but most of all i think jjos2xl excels on the sequencer, especially for live use. the next seq "immediately play" option is the holy grail for playing live, something akai didnt think of on their current mpc's, which, together with lack of proper time signature management, the lack of cursor and inability to program notes while the sequencer is stopped, make me hate that damn new sequencer. but of course the large ram and faster cpu enables plugins and tons of effects and most of all, great routing and automation, these i think are the strong points of the current line. plus battery on the live \ live 2. the sound on the new ones is very good, technically better than 1k\2.5k but subjectively i prefer the 1k\2.5k. anyway no comparison with the 4k.