When doing wrist curls or reverse one should dumbbell should move in plane perpendicular to ground or should it follow normal curved path as ulnar is shorter so weight moves toward ulnar side naturally. Which form is perfect?
You would be correct, as the ulna is shorter you do tend to naturally deviate toward that side.
However a major stabilizing structure called the Triangular Fibrocartilage Complex (TFCC) basically helps cushion and minimize any ulnar deviation seen at rest.
Honestly whatever's comfortable. Wrist Flexion / Extension is essential a pure sagittal plane movement. Try a straight or ez-curl bar, use whatever feels best -- risk of injury is low as this is a distal joint (so it's a much more isolated movement).
Other Exercise Options:
Use a variety of exercises to help to maximize the strength and contours of your hand, wrist, and forearm. This increase in the overall strength of your upper extremities will also help to improve your hand and wrist dexterity, which in turn can improve competitive performance in athletics, gaming, and even in day-to-day tasks.
Seated Wrist Hammer Curls In a seated position with your back straight, place your forearm on your thighs with your thumbs pointed upward. Use a 5-, 10-, or 20lb weight in a hammer position and lift it back and forth slowly for 3 sets of 20 repetitions. This will develop your brachioradialis muscle, which inserts at the distal aspect of the forearm at the wrist. Greater hypertrophy of this muscle will give more definition and balance of the forearm.
Seated Reverse Wrist Curls This is to develop your extensor muscles and is also done in a seated position with your forearms on your thigh, palms facing downward, with the wrist three to four inches away from the knees. Grasp the weight and extend the wrist fully. Do this for 3 sets of 20 and be sure to not lift the elbows from the thighs when extending the wrists. Keep the palms down.
There is never "one perfect form" in any exercise. One of the keys to progress is variety.
If you find one motion, and do it over and over and over, the benefits will suffer diminishing returns very quickly. You need to overload the muscles, both in terms of volume (sets x reps), intensity (weight), and variety.
And just to nitpick a bit; there are two planes perpendicular to the floor. The sagittal and the frontal planes. Only the transverse plane is parallel to the floor.