I don't inject anything anymore, but one drug I found far superior via the IV route was ketamine. Way less psychedelic/weird than IM ketamine and much more warm/opioid like. Also felt pretty dangerous considering how hard and fast it hits.
And cocaine and methamphetamine feel somehow cleaner when IV'd.
Another night and day difference is o-desmethyltramadol. It is a pretty meh opioid orally, but intravenously it felt much better.
I discourage anyone trying any of things unless they are already knee deep in that lifestyle already.