I think a lot of it has to do with the innocence of the drug. First, when you try weed you are usually young, so since you have barely any responsibilities or stress it makes it that much better. You can goof around, go exploring, watch movies, eat junk food, and that's completely fine, because all you have to do at that age is cruise by in school, and maybe work part time doing some easy task.
Then when you are older, a lot of the other drugs carry some more guilt with them compared to weed. If I'm smoking weed at night after I get things done, or even other times without it effecting what I need to get done, there isn't any guilt associated with doing that. But if I do the same with say opiates, I feel like I need to quit them, and that I shouldn't be doing them, which then takes away from the experience.
I think that weed is able to enhance more things than any other drug, and without making you completely unable to function semi-normally if need be. Like what other drug makes TV, food, bike riding, art, hiking, swimming, history, sex, music, and pretty much anything else imaginable better/more interesting. Other drugs may touch on a few of those things, but only weed can take such a wide array of things and truly enhance your experience with them. On it's own it's not the most euphoric drug there is, but it makes you happier than the other drugs, which is euphoric in its own right.