It really depends on the noise in your environment. I say this because a motor is involved, and they typically can throw a lot of noise around, and the cables to them are usually long, which makes them look like antennas.
Some background on those options:
0-10 V Analog Voltage
This case is where you generate a voltage with a DAC or, as Oli pointed out, a filtered PWM channel. The key here is that any noise voltage picked up will create an error in the motor's speed. So you want to have two things that make sure that voltage is where you want it: a low output impedance (the op amp should be good for this) and high impedance at the input (probably the case, but check to be sure). The goal is for the driver (op-amp) to be stronger than any received interference.
4-20 mA Analog Current Loop
This is used in a lot of industrial systems. Usually, industrial controllers have 4-20 mA I/Os, but microcontrollers generally don't. The idea is that driving 4 mA onto the loop is one limit and 20 mA is the other limit; since there's always at least 4 mA, the device on the other end can use this as a power source, and since it's current and not voltage, the impedance of the line is less important than the loop area in determining interference sensitivity. Since the wires are usually in a twisted pair, the loop area is minimized.
Short version: This is probably too complex. Look at 4-20 mA loop driver chips to see what you'd need to do.
0-60 Hz RS-485
RS-485 is a multi-drop version of RS-422, which is a differential signaling protocol. There are two lines in a twisted pair, and the driver drives opposing voltages onto the wires. The receiver then just has to see which wire has the higher voltage to see if it is a logical 1 or 0.
In RS-422, there is one driver and one receiver, and a 100 ohm or so termination resistor across the two wires at the receiver end. In RS-485, there are multiple drivers and receivers spread around, the wire goes point-to-point between them (a daisy chain), and there are 120 ohm termination resistors at each end.
In your case, you just need to connect a timer pin to an RS-485 or RS-422 driver, and hook it up like the RS-422 case. Then program the timer to generate the needed square wave.