Each costume has its own challenge, and whether it is easy to make or otherwise will depend on your resources (and sewing and/or prop-making skills). I personally feel the Refined Dress is the easiest as there is less sewing and easy prop-making. The hardest for me would be the FF VII Remake Default Costume due to the number of props needed on top of the complex parts and fabrics used. However, there are people who think that the default costumes are easier to make because they are better at prop-making, unable to sew due to lack of skills or absence of a proper sewing machine, or even find the materials and accessories for other costumes to be harder and more expensive to procure. Whichever costume you decide to make, your love for the character will usually enable you to go against all odds and make them a reality.