Definitley the greatest experience I've ever had with a drug was the first few times I did ecstasy. The warmth and energy and happiness that I was filled with can't be compared to.
It all depends on what type of person you are though, A meth euphoria and a Heroin euphoria are much different, I enjoy both but overall the euphoria I get from stimulants (mainly D-amp, rarely meth) are more enjoyable for me than opiates, yet combining them seems to reduce the stimulant effects too much and it also seems to reduce the opiate feelings too so it sucks, although I've never IV'd a true speedball.
LSD can provide with great experiences too but strong psychedelics definitely aren't for everyone.
God the list could keep going on, Thank Man for Drugs!