Why limit yourself to only one supplement?
You will get the best gains from 1-test. But I've never heard of anybody taking ONLY 1-test. Everyone also takes multi-vitamins, anti-ox, glutamine, creatine, fish oil, etc. You will also probably need something for recovery after the 1-test cycle, such as ZMA, Vitex, or Clomid.
I highly doubt that a "hardgainer" is going to get much "lean mass" from a clean diet. If you truly are a hardgainer, you need to be eating everything in sight.