In the end, Myst isn't the best adventure game (for my money, Riven holds that title), but it might be the most important. Some people love it (me), and some people hate it for ruining the genre by flooding it with cheap copycats. But there's no denying it changed the pc game forever, and you owe it to yourself to check it out.
A word about RealMYST: I own it. I recommend it. But it is not a replacement for the original. For one thing, the early 3d graphics are liable to look just as dated now as the pre-rendered images, plus the controls are a bit clunky, the sound has issues here and there, and, most importantly, the opening narration and most of the character videos have been replaced by new recordings - and the new performances are no where near as good. They sound rushed and noncommittal. If you've played Myst, then buy RealMYST as a sort of interesting special feature. It has a brief bonus age at the end, too. But if you haven't played either, I firmly believe you should first experience the game as it was originally presented.