A long time favorite of mine is the common betta, or betta splendens.
I think if there was an almost perfect aquarium fish, a betta comes very close. Best of all, a betta is a pet within reach of almost everyone who can set up a few gal glass jug or other water container. It's difficult in fact to find another animal you can keep for less investment then a single betta.
- You can keep them alone so you don't need to care for a whole shoal
- You don't need a real aquarium setup, larger tank and filter.
- They are fairly undemanding.
- They come in a variety of colors.
- They respond readily to people who feed them.
- They are readily available and inexpensive.
- The drawbacks being that they like higher temps so heating a small tank or bow is tricky
- IF you do want more, that becomes tricky as they have the pecking order among females and the males fighting.