The i1Pro 2 is a true spectrophotometer, and thus offers the most accurate readings for the widest range of displays at an affordable price. It is the closest one can come to a reference instrument for less than $1,000.
The i1Pro's only drawback is relatively poor low-light sensitivity. Thus, it is a nearly ideal choice to use in concert with the Display 3 colorimeter. Simply create an offset using the i1Pro 2 as a reference and then use that offset to correct the response of the Display 3. Then use the Display 3 as the color analyzer for the calibration session.
The i1Pro 2-Display 3 combo offers excellent accuracy, speed, and low-light sensitivity. It is also one of the most affordable options for professional use.