If someone has never used AAS before and they want to use only legal, non-steroidal drugs to help build muscle, here is a list that will make any natural bodybuilder blow up quickly.
SARMs: Closely related to AAS, but significantly safer...and when using low doses of certain drugs (such as 2-3 mg Ostarine/day) suppression of the HPTA is minimal, requiring no PCT. For the guys who aren't worried about suppression, moderate-high dose LGD (10-20 mg/day) is a great choice for more rapid gains in muscle mass.
MK-677: Just like several others have said, MK-677 is a great drug during mass-building phases, for multiple reasons. It pairs very well with all SARMs.
IGF-1 LR3: This may not appeal those guys who don't want to use steroids...because it needs to be injected, but when it comes to non-steroidal muscle builders, this is a certainly a good choice, especially when paired with a drug that activates the androgen receptor, such as SARMs.
Insulin: I am not really recommending this one (I don't think it is a good idea for guys who have no drug experience), but it needs to be mentioned, as it is a great drug when combined with SARMs and MK-677.
Creatine: This one's a no-brainer. Every bodybuilder should be using it, as it is dirt cheap and very effective. Most guys put on a good 5-8 pounds in the first 30 days. Although this initial weight gain comes in the form of water retention, it is primarily intramuscular in nature, which means it is going to look like muscle...and because strength shoots up proportionally, it will function like real muscle as well. In my opinion, anything that makes your muscles noticeably bigger and stronger for 15-20 cents per day is a shoe-in for inclusion. And for those of you who think creatine doesn't build actual muscle, think again. Creatine helps builds muscle through a half dozen different mechanisms, all of which are important for bodybuilders.
Leucine: Essential for triggering protein synthesis via mTOR activation. Leucine causes genuine, considerable increases in protein synthesis, which translates directly into gains in muscle mass. Like creatine, EVERY bodybuilder should be using leucine regularly...as in multiple times daily at a dose of 2-5 grams each, depending on the circumstances.