Flash is the most useless. While whirlpool is pretty much cut but on the water, it is still essential for game progrssion. Defog was useful for one area, but wasn't necessary (I didn't know about defog the first time I played DPPT
[thank god for aerial ace!]) Flash, however, is just mildly convenient for one or two areas a game and is rendered useless by having map from a walkthough/guide.