Well it is a really hard question!!!
It depends on the parameters on which you put on the word "pinnacle".
IF you are talking about difficult in keeping alive, then Altum Angels, Tropheus moorii- Bemba orange is one of my favorite color forms- (Tanganyikan Cichlids that are vegetarians who are notorious for a large part of your group just up and dieing for no reason!!!) Or as others have said any of the endangered wild fish, of whatever size, to get them breeding to keep them from going extinct!!!
OR if you mean a very beautiful fish who makes AWESOME displays: again Altum Angels, Discus, In a very large tank a breeding group of Tanganyikan Frontosa! They get pretty big at 16"+ and 5lbs.+, and the males get this huge golf ball size hump above their eyes, also they have BEAUTIFUL long flowing trailers on their fins that to me make them look like GIANT Glowing Blue and Black barred Butterflies!!! Also big groups of the small fish like Cardinals, Espi Rasboras, etc. make very stunning displays too! Some of the best comments I have ever gotten on any of my Planted Aquariums was when I had 25+ Cardinals in my first 55g!!! My Sisters Husband and his Brothers came to my Son's birthday party, granted they aren't aquarium keepers, and they couldn't get over the colors of the small FRESHWATER Fish! 1 of them had been Scuba diving in Fiji and said the Cards looked as good or better than any fish he saw there! Lastly a big group of Rainbows are hard to beat too! Their colors, shape, and activity level will keep you watching for hours! A 150g+ Display tank with Boesemani, Parkinsoni's, Turquoise, and Melanotaenia herbertaxelrodi "Yellow Rainbow" fish was the first real planted tank I ever saw, and it got me hooked on going planted and tearing down my African Cichlid tanks!
Lastly I think of the AWESOME Planted tanks themselves! Weather it be a Nature style or a Dutch style, a SHOW STOPPING, Heart Pounding display can take your breath away!!! Granted I have only seen a true few in person, but I have seen many pics of tanks, both in book and magazines or here on TPT, I truly aspire to be able to set one up one day when/IF my skill level gets to that point!!!