It really depends on your tank as most small fish are schooling fish that need a group of 5+ to stay healthy.
http://www.theaquariumwiki.com/10_Gallon
That's a good guide for a 10g tank, however if you have a 5g tank all that is acceptable would be a betta or a dwarf african frog. Anything less then 5g is suitable for shrimp or snails.
Personally if you have atleast a 10g tank, White Clouds are easy and pretty fish. You need a group of 5-6 but they are very hardy and like cooler temps so you don't strictly need the heater any higher then 70. They can live just fine without a heater but it's a good idea to keep the temp stable for fish health.