My personal favorite schooling fish are Neon Tetra. In large numbers they just look like a bunch of shiny fish moving all at once and their metallic like coloration makes them pop even more.
Tiger barbs are some of my favorite schooling fish. There is another breed of fish that I can't remember the name offhand and I can't even remember if they're in the schooling category. Tiger barbs are really cool and you have to remember to get them in groups of 5 or 6 so that they can hang out together. They can be a little aggressive so that's something to watch out for unfortunately.
For freshwater fish, I would say fancy guppies. I love the rare colorations they can come in but they are hard to find. I had a few that had neon green with bright orange, they were stunning.