The easiest salt water fish to keep would definitely be fish from the grouper family, the damselfish family, triggers, lionfish, eels. These are the fish that people have been keeping since the beginning of salt water fish keeping, the 1970's, 1980's, when people really started to see this as a viable hobby. Only recently have we had a lot of success with some of the more delicate tangs, gobies, clownfish, angelfish, and butterflyfish.
But within those groups there are hardier fish and there are more sensitive fish. Again, if you are a beginner, I would try a nemo clownfish or pakula clownfish, the blue damsel, the bicolor damsel, four-stripe, or three-stripe. You can go with any of the triggers. Just keep in mind some of the triggers are very aggressive. I like Niger triggers and blue throat triggers. Lionfish are great, but they get big and they can eat their smaller tank mates. But, once you get a good one they're very easy to keep.
Groupers are very, very easy to keep. They have large, thick scales, very resistant to diseases. They'll eat anything. If you can't get a grouper to eat, there's something drastically wrong with your aquarium. Then some of the wrasses, lunare wrasses, six-line wrasses, yellow coris wrasses, those guys are pretty easy to keep.
What I would do is try to get one fish from each family to prevent aggression, because usually fish are more aggressive towards fish of the same family. If a fish looks the same, has the same color, the same body shape, usually they eat the same foods and require the same habitats in nature. So, the fish have a built in aggression towards those fish.
If you go with a hippo tang, a yellow tang, different body shape, different color, usually those fish are very compatible. If you try to go with a purple tang and a yellow tang, same shape fish, they're going to fight like crazy.
So you can go with one small grouper. Keep in mind they get very large. You want to make sure that the grouper that you go with is going to have a large home eventually. A niger trigger, a small damsel, a lunare wrasse, and a small lionfish, and as these fish grow together they usually recognize their tank mates not as food. I've seen lionfish being housed with small fish from the start and they don't see their tank mates as food. It's just one of those weird things.
So, mix up the different groups of fish and stick with the groups that I mentioned. Stay away from butterflyfish. Stay away from angelfish and most of the tangs; except for scopas, sailfin, hippos, yellows. A lot of them you should leave to the more dedicated, experienced reef hobbyist.