What you're probably want to look at is designer drugs. It's a fascinating subject, essentially designer drugs attempt to recreate the effect of regular drugs while attempting to circumvent issues of legality and testing by using no regulated active ingredients. Basically a drug test will test for specific active ingredients, if the drugs you're taking don't have those ingredients then no positive test.
But still risky all same, since new ingredients are constantly being classified as illegal, and some ingredients might be metabolised by your body to produce something that will be detected in a test.
But if you must still do drugs, then it gives you the lowest probability of getting a positive drug test.
Now I'm not a drug test cheating professional, so if you do go this way make sure you do your own research about what the active components are and more importantly what type of drug test you'll be taking i.e. what the drug test will be testing for.