The best batter I have ever tasted was (suprise surprise, I know) a Heston Blumenthal recipe. It's absolutely awesome. Here it is:
200g plain flour
200g rice flour
(or just use about 350g of ordinary flour instead of these two)
1 tsp baking powder
1 tbsp honey
300ml vodka
300ml lager (Kronenbourg 1664 works well)
2-3 litres groundnut (peanut) oil
Table salt and freshly ground black pepper
You can use ordinary oil for this but try to get the oil hot without burning it.
Basically just mix all the wet ingredients together (apart from the beer) and the dry ingredients together. Then combine the two and refrigerate. Put this in the fridge for about a half an hour or until ready to cook your fish. When ready to cook, dust the fish fillets with flour, remove the batter from the fridge and open the beer and mix it into the batter, keeping the bubbles if you can. Coat the fish in the batter and fry until golden brown. Another Heston trick is to drizzle extra batter over the fish while it is cooking, to give it more crunch.
This recipe is pretty foolproof and is absolutely delicious...