Opiates make me feel "the best", and I've never had H but I LOVE LOVE LOVE Oxy However my favorite would be shrooms. While they aren't necessarily the most "fun", they always teach me such invaluable lessons about myself, and that really can't be trumped by any other drug.
I'm an amphetamine fan, I love the happiness it gives with being so productive. Amphetamine works mainly on dopamine though, so if you don't really like speed and only take it to be productive you're not gonna get the same enjoyment