I'm enjoying this thread!
I find the term "Centerpiece" or "Showcase" to have a high degree of subjectivity. For me, a centerpiece fish or showcase fish should check the following boxes:
1) The fish is something the common unknowledgeable observer would find eye-catching and beautiful; it should not be simply rare (Gem Tang, Sohal, Tiger Angelfish, Conspic, etc).
2) The fish should be large (7"+), which means most fairy wrasses and pygmy angelfish are out of contention
3) It should move majestically, like a princess entering the ball
My favorite centerpiece fish are as follows:
1) Yellow Belly Regal Angelfish
2) Belize Queen Angelfish (they have that deep teal throughout with the orange and yellow peaking through)
3) Blueface Angelfish
4) Emperor Angelfish
5) Garibaldi Damsel
6) Naso Tang (Blonde and Regular)
7) Clown Tang (too aggressive but beautiful)
8) Achilles Tang
9) Desjardini
10) Yellow Tang (I can go either way on this one)
11) Bennetti Butterfly
12) Meyer's Butterfly
13) Semilavartis Butterfly
14) Trifaciatus Butterfly (if you are lucky enough to see an adult, it is a sight to see)
15) Aussie Harlequin Tusk
16) Cuban Hogfish
17) Adult Bi-color Parrot Fish (if you have never seen one, you are missing out!)
Honorable Mention (which is where many of my favorites exist):
1) Asfur Angel (when the light hits an adult male just right, it is crazy beautiful)
2) Powder Blue Tang (The color pattern of the Achilles is just more striking)
3) Blue Hippo Tang (it is iconic)
4) Majestic Angelfish
5) Bandit Angelfish (it looks like it is made from expensive felt)
6) Most black, yellow, and black butterflyfish (Falcula, Double Saddleback, Tear Drop, Tinker's, etc)
7) Meridithi Angelfish (the males have a cool facial pattern)
8) Rock Beauty Angelfish
9) Male Swallow-tail Angelfish ( When they have the blue sheen with the yellow highlights, it is so pretty)
10) Too many more to mention!!!