The easiest fish to care for in a tank is going to be the one that has the smallest fins, and the easiest fish to take care of is going to be the one that can swim up to your eye. This may not make much sense, but it makes sense once you get started. If you have fish that can swim up to your eye and can survive in that type of situation, then that fish is the easiest fish to care for in a tank. The problem comes in if you have fish like guppies or denies, that cannot survive in such an environment, even if they are small.
Best Fish for a Bowl: White Clouded Minnows. Easiest Fish to Care For In a Bowl: White Clouded Minnows. Best Fish to Take Care Of: Betta Fish, unless you have a filter, live plants, and/or a power head attached. I don't know why this is so, but it is true. This species of fish will tolerate most of what life throws at it, and will even thrive in a low ph range aquarium.
Discus Fish - These are some of the easiest fish to take care of, if you don't mind living with small water conditions, and a slow movement's pace. Discus fish will live in most water conditions and will even do better in some deep water corners. They are very forgiving and are capable of adapting to new water conditions, if you change your water conditions slightly. This is what causes discus to do well in freshwater tanks, because they are naturally tolerant of small changes.
African Cichlids: These fish tend to be very hardy, and are often sold as a starter set. African Cichlids are great for tank first time owners and also great for tropical fish, as they are very hardy, and can handle either high humidity or low humidity. I would suggest that you pick up one of these fish, as a starter set. The color and white patterns are cute but not very hardy.
Red Fish: Red fish are by far the easiest fish to care for, and they can tolerate most water conditions. You should give them some space, as they will enjoy it, and they like to jump and dine on bugs. As long as you leave enough room for them to do so, they will do fine. They will also thrive in tanks with aggressive fish, and they will compete for food. Make sure to check twice, that you have no other fish, or any livebearers in your tank.
Other Fish: Some other fish that are great for beginner aquariums are places, zebra danio, and albacore. All of these can survive in a wide range of environments. Just make sure they have plenty of space, and plenty of food, as well. This will help you get started with proper tank maintenance.
Once you get to know your fish, you can start trying out different fish and introduce different ones to see which ones will do best in your climate and pH range.
