DXM + Weed is amazingly comfortable and euphoric.
Opium + Clonazepam is one of my all-time favorite combos, feels like a warm bath of pure dopamine for your brain. Really any opioid and benzo is nice, my go-to combo. H and diazepam is another noteworthy one.
Methylphenidate + Heroin is nice as well, I imagine that's what a speedball is like. Few lines of dope and a crushed up 10mg IR every so often. IR amphetamines are also good and I like them with other drugs, but Ritalin is my favorite to mix with dope.