Testosterone, as the natural product drug and one of the most widely used anabolic steroids, is the most convenient choice for a reference drug to which all others will be compared. And while it is entirely possible to construct maximally-effective steroid cycles without employing testosterone, most do not do this, but instead use testosterone as their foundation. Either approach can be entirely sound.
As a bodybuilding drug, testosterone is almost always used as an injectable ester, due to poor oral bioavailability and the impracticality of high dose transdermal or sublingual delivery. Testosterone also is provided as an injectable suspension. Discussion here is in reference to these injectable preparations.
Pharmacologically, testosterone acts both via the androgen receptor and via other means. In practice, it is found to combine synergistically both with those anabolic steroids categorized as Class I and those categorized as Class II, and therefore is described as having mixed activity.
Particular properties of testosterone that are of note include that it converts enzymatically both to dihydrotestosterone (DHT) and to estradiol (the most important of the estrogens.)
While with normal levels of testosterone and normal enzyme activity these conversions are in fact desirable, with supraphysiological testosterone levels caused by drug administration they can be undesirable. DHT is at least three times more potent (effective per milligram) than testosterone at the androgen receptor (AR): therefore, in those tissues which convert testosterone to DHT, there is effectively three times as much androgen as elsewhere in the body. Thus, whatever level of androgen is experienced by the muscle tissue is effectively multiplied threefold or more in the skin and in the prostate. This can be excessive.