Motor/driver for smoothest and most constant speed
Without more info it's hard to do other than generalize, but for low torque ripple a permanent magnet sinusoidally wound BLDC (synchronous motor) with position feedback from a resolver or high resolution encoder and a driver that creates true sinusoidal excitation (via conventional/FO control using SVM or 2 phase current feedback PWM) will give smooth torque and low torque ripple.

Avoid trapezoidally wound BLDC motors, hall sensor feedback, switched reluctance motors, and sensorless commutation schemes (unless you know they perform very well with your particular motor in your speed range of interest.)