Wow, that's a tough question. You may as well ask what my favourite song is.
@bgarthe , you put up a good cardinal tetra picture. It may be common and lack the aura of rarity, but it is a beautiful creature that's good fishwatching. LIke the black neon or the glowlight tetra, it's an essential to me. They're popular for a good reason.
Hyphessobrycon melanostichos scores high for me. They're selling them with the annoying translated Mandarin name "Imperial Lapis tetra", but in spite of the bad poetry, they're worth exploring.
I like every Alestopetersius African tetra I've ever met. The most common one is A. caudalis, which I can't seem to get my paws on even though it is farmed. I have young Alestopetersius smykalai here, but the jury's out on how they'll develop. This red one pictured may be A. brichardi. They lived close to 10 yeas here and could have gone longer without the stress of my long distance move, I suspect.
Once, before digital photogaphy, I had some Rachoviscus crassiceps. They were little reddish bulldogs, with fantastic faces. They were easy to breed, too. I've never seen them again and didn't get photos, but they were outstanding. Some might say they were ugly, but they had more character than most Cichlids. Alas, they weren't pretty enough to return.