Why are seals top of the list for sharks?  Why not larger fish?

Do you mean why do sharks prefer eating seals to big fish?

If so I don't think they do. There are a lot of shark species out there and between them they cover a lot of dietary variation (from plankton, to larger invertebrates, fish and marine mammals). Some sharks are pretty general in their feeding habits (i.e. they will have a bite of anything) but many are quite specialised. I'm pretty sure they are all carnivorous though.

Seals probably have a high energy content due to their blubber so that would make them - energetically speaking - an attractive food, but species like teh great white also aet other marine mammals (e.g. dolphins) and fish (including smaller sharks).

Whilst all sharks and rays are carnivorous the group as a whole has a very varied diet. No shark species preys exclusively on seals and any species large enough to take seals, as Alistair says, will also prey on bony fish and even other shark species. Seals take a lot of energy to catch as they are fast and agile. They are therefore only hunted due to their large reserves of blubber that make it worth while in terms of energy expenditure.

Conversel, many species of shark are durophagous i.e. they have teeth that crush shelled invertebrates, such as crabs. Three of the largest shark species- the Megamouth shark, the Whale shark and the Basking shark, feed almost exlcusively on plankton- they also take small fish and other small organisms that get in their way when scooping up plankton.

Great Whites are comparatively fussy eaters whereas the tiger shark has a dentition that is able to cope with eating anything it finds meaning that they are generally not in the least bit fussy. One tiger shark was found with a piece of a suit of armour in its stomach!