I wouldn't say either of these speedballs that I've tried are the BEST... but I've tried:
Dilaudid and ritalin in the same syringe. that was actually quite nice. I beleive i was on valium (oral) at the time if that makes a difference... prolly not much but.. hah.
The most epic syringe I ever prepared was like a 15mg oxy, 4mg dilly, prolly .15g heroin and a 10mg ritalin. That was quite the rush. That night mixes between dilly and h, ritalin and oxy, and other combos prolyl were made but...
Once I did a shot of 12mg dilaudid while still high on over .5g meth IVd on one shot @_@. I really like doing dilaudid either while high on or coming off of meth. I feel like it may strengthen the opiate high, plus you're not doing both drugs ar once so you're high longer!