Generally when people look for the most efficient high, they use the "Bioavailability" property. This is how much total drug gets into your system over the duration of the high and is expressed as a percent (with IV being 100%).
Bioavailability/Half-life MEGA Thread
However BA doesn't tell the whole story. The maximum concentration and time to maximum concentration (rush) is also an important aspect of how you dose your drug. Having a high maxC might result in a lower BA so you will get higher but for a shorter period of time.
So for oxycodone if you want the most out of the drug to get the highest BA you should take it orally, however if you want a better rush and to get a little higher just for not as long you should take it nasally. Vaporizing oxycodone requires lots of processing to extract it from the pill and then would likely result in even faster onset but much lower BA though I'm not sure.