Generally however a good rule of thumb is:
Buy your test kits that originate from the area in which you live.
You live in Europe go for the EZ-Test or the Chemical Solutions combinations
You live in Australia go for the Australian Testers
You live in North America go for the Dancesafe ones
Each region generally has a worse problem with some contaminates of what should be pure MDMA + filler pills. The US has DXM and 2CT7, Australia has Ketamine and some PMA. The Dancesafe testers are good for picking out DXM whilst the Australian ones have a test for Ketamine and PMA. EZ-Test provide the most comprehensive solution which allow you differenciate amines.
If you only want to buy one bottle and are in Australia or Europe, definantly get the standard Marquis reagent. Follow the rule "If it doesn't go black - throw it back" and your chances of coming to grief are reduced. If you are in the US go for the Dancesafe tester.