NITRITE
In my opinion, the most useful/important test kit out of ammonia,nitrite and nitrate. I know ammonia is more toxic, but biological filter media generally quickly can mature to process ammonia compared to processing nitrite which can take longer. I have tested tank water in the past for other people where ammonia is zero but nitrite is very high. Therefore if you are choosing between ammonia and nitrite and can only afford (or dare i say it be bothered to have only one of these kits), go for nitrite.
See following links for info
http://www.thetropicaltank.co.uk/cycling2.htm
http://freshaquarium.about.com/cs/bi...rogencycle.htm
http://members.optushome.com.au/chel...rification.htm