MPC 2000s can be picked up cheap and they still sound wicked...

as for synths... jeez now your asking!!!
The synth with the most amount of modulatyion capabilities is what you want. TBH the question is like asking - "Which car is best for roads?"

My advice is to check out as many synths as possible on manufacturers sites, listen to demos and try and make up your mind that way.

Alternatively i found these synths to be fun to use.

Novation K Station
Nord Rack 2
Alesis Ion