For me, far, FAR too many fish to possibly call one out as the prettiest.
I could type for pages just on the New World representatives of the family cichlidae alone, then move on to Africa with the same family - that would take pages and pages, and then move on to charcins, the live bearers.
Even making a top 30 list is a fools errand. (for me), if I cared to take the time it would be more like a "pretties 1000"
I mean look at a good red top Afra, or Zebra 'Chilumbae' male in full dress - that's without even thinking about Victorians.
Yeah - fools errand.