Well, injected testosterone at low amounts.
This is why for many years, there were many scientific studies showing that anabolic steroid use did not improve muscle mass or aid athletic performance. They studied low doses, which usually do not do much in these regards, and are too inconsistent to give statistically significant results in small studies.
Dosing such as used today by most interested in muscle gain is typically at least 500 mg/week. Injected testosterone at that level certainly does a lot for muscle gain, but doesn’t sound like what the OP wants.
(I do expected that by injected teststerone, you were meaning small amounts rather than bodybuilding amounts.)
Indeed I do find that testosterone boosters aren’t much for muscle gain. I recommend Alpha Male for feeling better, and perhaps lifting better as a result of that feeling, or perfoming better in other ways, rather than for direct anabolic effect.
For the OP I would suggest that following the best training and dietary advice is generally the best way to improve results outside of supplementation, and for supplementation the first thing to look at really is workout nutrition. If you do feel symptoms of low testosterone such as lack of libido, Alpha Male might be an excellent supplement for you.