From the 2D "HD" FF games on iOS FFV has the prettiest sprites. FFVI look more like pixel art to me TBH. Can't really tell much about Dimensions: the sprites on the Wiki are terrible screenshot rips so it's hard to tell for me.
Although I really dislike sprite work on the iOS FF ports: males look angry all the time and that is so out of character for the likes of Bartz for example. OTL