Well it's a year later from when I first started this thread... I now have 8 dragons at level 15. They are: Fire, magic, light, Diamond, Infinity, Dueler, Beast, and Rainbow Sentinel. I have 27 other dragons at level 14, 13, 12, and 11...

I've bred 3 different prime dragons who are not super impressive to me. The third one I got when trying for something else. I'd say feeding to level 15 should be based first on who your best battle dragons are. To me that's a champion type dragon (or two) and a diamond (if you have it). On one of my other lower accounts that means I've fed up my Passion dragon. Others should feed up Metal. The sheer amount of food needed to get a dragon to 15 is time consuming. My preference is to have every different type of dragon I own to at least a level 7, ones I like to a level 10. Good battlers and stupid dragon tales requirements are the levels 11 and up.