QFT, but also note that although "excellent" quality is the best you'll see, having a serum reach that point doesn't mean it's the "best" - make sure the purifier says it can't be made better before you try to put it to use.Make sure to ALWAYS, ABSOLUTELY ALWAYS, WITHOUT FAIL, use the highest quality serums.
This depends on what you want to duplicate. If you want to turn a princess from a freshly-scooped hive into a clone of a max speed/territory/tolerance/etc princess, you can either spend forever injecting serums into it, or you can just let it breed a few times with a stack of suitable drones to bring multiple traits into line at once.Using bee serums is actually a lot easier than manually breeding duplicate bees once you get the hang of it.
If you want to bypass the annoying hybrid stage involved in mutating to each new species, serums are your best friend for skipping through that process.
I wrote out this post explaining the genetic machines, once upon a time. Might be useful. Reading this thread will also answer a lot of questions related to bee-keeping.