That is based on preference and almost impossible to answer. I would say the following are up there:What's the coolest fish to have in a large tank by itself?
- Large black piranha (15in or bigger)
- Large aimara wolf fish but this can potentially have tank mates
- Fahaka puffer
- Aba aba knifefish are really cool
I believe there are a number of cichlids that are high on the list but the above are the ones I have scene or care about.