Like someone else said, I can't pick just one.
First of all, I love cardinal tetras. I had neon tetra disease devastate a tank of mine and, as always, my cardinals remained resistant. Haven't lost a single one and that's pretty amazing considering how many others I've lost.
Head and tail light tetras are excellent looking. In bright light, those two reflective spots are gorgeous.
I feel that x-ray tetras are highly underrated. A school of those will really make the color of cardinals stand out.
Black tetras are not only beautiful, they're bigger tetras and not as skittish because of it. Mine love to see me. And they don't have the aggression of Buenos Aires tetras.
Congo tetras are just amazing. Again, bigger tetras and peaceful.
I've had a hard time finding rosy tetras, but I prefer them to serpaes.
There's a special place in my heart for Von Rio tetras, too.
If you've never seen red phantom tetras, do so.
Penguin tetras are hard to find, but they are so neat in that the swim diagonally across the aquarium. Adds an extra dimension to the rest of the fish.
That's all I'm saying for now.