Well, there's two main brands, Androgel and Testim. I used Androgel for 5 months -- the results for me were lukewarm at best. It is easier to apply than Testim, but not as powerful, according to the lit.
Try googling 'Androgel vs Testim' and read several of the results/studies.
Other people here have gotten good results with Androgel. Most will, I did not.