The only pill that I love to do intravenous is Methylphenidate. I didn't like intravenous morphine, 30mg IV makes me feel too drowsy and sleepy, then I can't enjoy the euphoric rush, and I know, oral morphine has a poor bioavailability, BUT 60mg oral is enough to make me feel warmth, happy, talkative, relaxed and proud. The only opiate that is nice intravenous or intramuscular is Tramadol (my personal opinion, friends).
Midazolam (IV or IM) is fine, but oral xanax is so much more funny,
I'm not a fan of intravenous route, the only drug that I loved in that way was Ritalin, thr kick in is so much different from insufflation and oral absorption. Swallowing the pill or snorting it makes me feel strange, antisocial and anxious. Maybe the metabolism have an influence on it, or just the speed that the dopamine are concentrated between the neuronal gap. I don't know, but intravenous Ritalin feels like another drug for me.