Good question.
Best bang for the buck would be the ZC motor. It bolts right in with no shift linkage or motor mounts to buy, can be had for $500 if you shop around. Then with new hoses and belts, etc, you can have it intstalled and done for around $700. Decent power, easy cheap swap.
BUT.. the B18a1 or b1 are a close second place. You can find them, complete with enginge, transmission, and ecu for $800ish. More torque than most of the D series motors, 140ish hp, easy swap. But on top of the $800 you've got to buy axles, knuckles depending on the car, mounts, and shift linkages.. Its easily another $600 on top of the motor cost.
Then, the GSR (B18C1). This is a great motor. Great power and torque, almost as easy to swap in as the b18a1. But its more than twice the price, these go for $2000+