What are you interested in? Cichlids? Catfish? Bichirs? Knifefish? Personable fish that will greet you? Aggressive predators that will snap food out of your tongs? Surface fish or bottom dwellers? Kitchen sinks or biotope setups?
You've got no shortage of options with that volume. Even if you want something totally unfeasible in a 150, like an RTC, there are smaller alternatives that behave similarly. If you have any preferences in mind, that would make narrowing things down easier.