With that said the strongest legal oral steroids (for mass & strength) ever released were...
1.) M1T: The most potent oral steroid EVER produced under any circumstances. Trumps all prescription steroids in terms of real-world potency. It is now a controlled substance.
2.) SD: The 2nd strongest oral steroid ever legally released. Also stronger than any prescription steroid ever released, incluidng Anadrol, d-bol, etc. It is now a controlled substance.
3.) Dimethazine: The 3rd strongest on the list. Currently legal. Like the first two, also stronger than any prescription oral steroid ever released, with the possible exception of Bolasterone. Once a prescription steroid in Italy & mexico, it was clinically tested by Big Pharma for myotropic effect (growth potential). It was compared directly against Anadrol, Test prop, Dianabol, Winstrol, and Methyl-test. It was shown to be more powerfull than all of those drugs for inducing muscle growth when evaluated on a mg per mg basis.
Honorable mentions: 1-Alpha, Desoxymethyltestosterone (phera-plex), Methylstenbolone. My apologies if I forgot any of comparable potency.
Of course, there are many other legal orals that many people like, such as Epistane, but they are more on par with drugs like T-bol, Anavar, or Winstrol, although some of them due have significant muscle building potency. Epistane would be one good example--it has anavr-like effects, but builds significantly more muscle, per mg.
As with any steroid, selection should be dictated by one's goals.