Stick To Training A Balanced PartyThere are really only two ways of training your Pokemon in Pokemon GO, just like in the Mainstream Pokemon Games, either by committing all of your time, Stardust and Candy into one Pokemon or in a balanced distribution among a number of your Pokemon. Both ways have pros and cons to them, but the far more effective way is committing to a balanced team.
When using a balanced team, there are plenty of more advantages than using just one Pokemon. First off, a more balanced team will often mean you are evolving more Pokemon, which, in turn, means you are furthering your pokedex (the main concept of the gaming series), as well as growing in trainer Level. A more marginal team also means you will have many equally great Pokemon to take on Pokemon Gyms and Raid Battles with. You will also have plenty of Pokemon to leave at a number of Gyms, which means you will easily rake in more PokeCoins for defending them.
However, there are a few setbacks for this way of training. For one, you will probably only have a few Pokemon that will be as high as some Pokemon you see left at other gyms. The process of raising a number of Pokemon also means it will take more time for you to be good enough to take on Gyms with persistent winning. It will even take more of your resources, including both Candy and Stardust, to do this form of training compared to raising one single Pokemon, at least if you are doing it right anyways.
Slow and steady usually wins the race, and that is why this is not doubt the more effective way with going about Pokemon GO. Raising one Pokemon might get you the greatest Pokemon of Pokemon GO, but it will not give you the constant success as a balanced team will one day get you.