The best combination of a stimulant with a downer I've ever had was with meth (desoxyn 5mg x 1) + opana (~20mg snorted over an hour). The euphoric properties of each just grabbed hold and ran. I was in a warm blanket of opioid clouds, with a mind that would allow me to enjoy each second while remaining semi-productive (played my synth for a good couple of hours with a fat grin on my face). I found amphetamine to work ok with oxycodone as well, but was extremely disappointed by the combination of methadone and amps. Felt like a waste of both goods to be honest.
Meth + oxymorphone + ketamine + fat bowl + cup of coffee + a few cigs = somewhere close to heaven. Now, never had the pleasure, but NMDA-antagonists go fantastic with opioids, as well as weed. The combo involving a various form of amphetamines with NMDA-antagonists seems to always be a treat too.