my experience it was Giant Snakehead then Flowerhorn. My big brute in the avatar was 1.5" when I got him and then 12" within the first year. Crazy growth. Now 14" and 3.5" thick. Obviously been a long time since I had SH but they grew rediculously fast, I miss keeping them.
In my experience? RTC can grow almost an inch per week on feeders. Then gator gars half that (of an RTC) on feeders, too. I bet the arapaima would have an astounding growth rate. I don't know about the rest but I wish my GG can grow as fast as even a pbass.