Juddering Wiper Blades - Incorrect Wiper Arm Angle

Assuming your rubber blades are not worn out, and your windshield is 100% uncontaminated and smooth, the other cause could be your wiper arm's angle. This is most common on older cars. 

Imagine using your glass squeegee to clean your glass panel. 

As you pull the squeegee down, the tip of the rubber blade faces UP. 

This gives you a clean, smooth sweep downwards, and you have a dry and clean glass panel. 

Your wiper blades operate on the same principle. 

As the wiper arm goes upwards, the rubber blade's tip faces DOWN. If the tip faces upwards, that rubber blade won't go upwards smoothly, and it'll judder. 

When wiper blade reaches the top, the rubber blade should immediately and noiselessly change direction. 

On the downward stroke, the rubber blade's tip shd face UPWARDS as the wiper arm pulls the rubber blade downwards. 

If your metal wiper arm is bent, the above might not happen, and you'll have juddering issues. 

To check, look at your wiper blade at its resting position. 

Ideally, the wiper blade must be exactly perpendicular to the windshield, meaning its 90degree to the windshield. If it's not, then use pliers and twist the arm until the wiper blade is perfectly perpendicular to the glass. 

BMW and Mercedes even have specialized wiper arm-twisting/alignment kits for this purpose.