Any of em.. if Dre used a $5 keyboard on a track that went platinum, everyone would be looking for the same thing. Personally I like to use gear that no-one likes.. If folks say something is wack, I gravitate towards it.. I'll make use out of any instrument.
If by keyboard you mean sound maker, I would suggest getting 2 - Yamaha TX81Zs and a midi keyboard controller. The TXs are old skool and cheap. $75 to $100 each. Nice sounds and lots more available for download for free. With a software editor you can make it even better.