No-intro romsets is the best since those are unmodified and taken from a clean dump. They do not have anything else added to them that is not default in the original game.
As @Clyde says You can use Romcenter or you can use Clrmamepro with the .dat files from no-intro website and then filter them out for 1G1R also known as 1 Game 1 Rom. This process is called (Parent-child).
This will make sure that you don't get clones and only keep one game from one specific region. Ex. Sonic the Hedgehog (USA) instead of getting both Sonic the Hedgehog (USA) and Sonic the Hedgehog (JAP) in your Mega Drive collection.
There are a couple of tutorials out there on the internet that you can use.