Pelagics

Large schools of Pelagic fish (defined in Wikipedia as fish that "live in the water column of coastal, ocean and lake waters, but not on the bottom of the sea or the lake (and) contrasted with demersal fish, which do live on or near the bottom, and reef fish which are associated with coral reefs") are one of the great sights of scuba diving, especially when a large school is made up of large fish. My first sighting was the previously-mentioned barracuda battery in Thailand while I was still learning to dive. I saw even more barra batteries in the Similans, that fantastic dive location in the Andaman Sea off Thailand.
Funnily enough, one of the greatest pelagic displays I've ever seen was at Harbord in Sydney on my 114th dive. By sheer luck for our small Dive 2000 group a big school of yellowtail kingfish, each fish averaging about a metre in length, suddenly arrived and started terrorising a school of yellowtail scad. It was an amazing thing to see and we were right in the middle of this hunting frenzy. My inveterate buddy, Liz, managed to get the event on video (see D2K website).
Another great site for pelagics is Lady Elliott Island, where schools of giant trevally cruise around and there is a resident group of manta rays. I've seen schools of large kingfish and mulloway at Julian Rocks and Fish Rock but I'm yet to see a pelagic shark, unless Port Jackson sharks are counted as such. They spend most of their time in very deep water but we see them in winter when they come to the reefs around Sydney to breed. Another shark I've regularly seen in Sydney, specifically at Fairy Bower, are the juvenile dusky whalers, who hang around the seagrass beds in summer but eventually grow up and become pelagics in the open ocean. Most of the grey nurse sharks I've seen have been at Fish Rock (a place where you're guaranteed a GNS sighting) but, even though these babies are big, they're not pelagic like great whites and other dangerous sharks.
The Yongala wreck in northern Queensland is another pelagic wonderland. The wreck is effectively an isolated reef in the middle of a 'sandy desert', so pelagics regularly come in and pick off hapless members of the resident fish that have established a home on the remains of the steamer that sank in a cyclone almost a 100 years ago.