1)As a single drug on its own, my vote would have to be a toss up between DXM and MDMA, although I've gotten great body-highs off of LSD and 4-Aco-DMT a couple times. Combination when done properly, however, can produce euphoria far greater than the sum of their parts; see below.
2) Yes, very.
Here are my most euphoric combinations in terms of body-high:
LSD + MDMA: I tried the infamous candyflip for the first time recently and it really is an epic combination. I had sooo much fun that night, but it isn't the most euphoric combo I have experienced, contrary to what I was told to expect.
Mushrooms + Ketamine / LSD + Ketamine : I tried these combinations at a music festival this past summer and found them both to be awesome. Great body highs, cool visuals, and some of the best music enhancement I have experienced. I'm grouping these two together because I found them to be very similar; moreso than mushrooms and lsd on their own.
Mushrooms + DXM + Benzo: This was my first psychedelic combination, and still my favorite. Time your dosages right (Shrooms at T+1.5 is the key) and words will not be able to do justice to just how amazing this combination can be. Expect ego-loss, out of body experiences, and awesome CEVs bundled into fiveish hours of indescribable bliss. Here's my retrospective on the combination.