The most intense rush I've experienced was smoked 5-MeO-DMT
However, the most favorable (meaning I enjoyed it, also intense) rush was most certainly IV fentdope (like someone else mentioned above). Unfortunately a few people I knew OD'd and didn't come back from said substance. This was by far the best rush I've had though.
Other rushes I've experienced include:
IV > Heroin, FentDope (Heroin fentanyl laced), Oxycodone, Morphine, Cocaine, and combination Heroin + Coke
Oral > Dilaudid (SUCKED, wish I had the needle), Methadone, Oxycodone, Hydrocodone (and high dose CWE, Fun!), Morphine, Codeine, Suboxone/Subutex (of course sublingual), Ectasy, Shrooms, LSD, Methamphetamines, Benzos (Klonopin, Xanax, Valium), 5-MeO-DipT, Alcohol
Intranasal > Heroin, Oxycodone, Morphine, Cocaine, combination Heroin + Coke, Suboxone/Subutex, Ectasy, Methamphetamines, Ketamine, Fentanyl (what a lucky find that was, it was before I started IVing)
Smoked > Heroin, Opium, Weed, Screened Kif from my harvest, Salvia, Cocaine/Crack, Meth, combination Heroin + Coke, 5-MeO-DMT (omg, I could write a short story on this experience)
Plugged > Ectasy (not a big fan of this ROA)
I'm probably leaving something out, but that's what I can tell you off the top of my head without putting strenous thought into it!