There goes that word again, what is the "BEST". Best for what, what are your needs, what can you afford, what do you want, what style of music are you doing (I assume Hip-Hop), but the best is really tough to say. You got an MPC4000 I'm assuming so you really don't need much else.
Everybody and their mother has a Triton. The Fantom yeah it's okay I guess never really heard it's sounds. I have a Motif ES but I can't really suggest you getting that either. Get you a good midi controller like the Pro 88 from MAudio and you can control all your sounds from off the MP. Or even get something even cheaper, it depends on how creative you are I can't really tell you what the best is unless we know what application your using it with and what your goals are.