It's kind of a weird question as to which is "most important", as I would argue they are all important for their own reason. You're not going to generate any real power without the use of your legs or hips, and you're not going to land the ball where you want without the proper arm and wrist movement.
However, I would say that your legs are your foundation to your forehand loop, but i'm not sure if that is the "easiest" first step to master. For example, you could load up your legs and get a full torque, but if you can't make good contact on the ball, you're just wasting your energy lol.
I know in my personal journey, I already had a decent arm stroke motion from when i was a beginner. but i had a bad habit of standing too tall, which essentially meant I was not utilizing the power of my legs. I had to spend a decent amount of time learning to stay low on all my shots.
So if I was a beginner and I already knew how to make contact with the ball, i would focus on staying as low on my shots. When you are "low" this means your knees are bent, and you have power built up into your feet and legs. you can use that power to rotate your body and swing through the ball with a good contact. Again, its hard to say what is the most important, because the full mechanic is : load power into your feet, calves and quads by bending and twisting (to the right if you are a right handed player), start to unload that power by pushing off your back right foot, then twisting your hip and torso in a clockwise motion (if you're right handed), while also keeping your arm mostly relaxed until its time to snap at the elbow before the contact on the ball, and then squeeze your hand and the moment right before the contact of the ball, completing your follow through forward or up to your head.
But if you can't make consistent contact on the ball, you need to spend some time just "driving" over the table before you can effectively add in the mechanics of the full body power unleashing.