I love Myst III: Exile since that one has Brad Dourif in it. I only played Riven through once, ten years ago or so... when I tried playing it again on XP, I couldn't get it to work anymore so I sold it. *shrug*
A pretty remarkable first person adventure game/Myst clone is "Amerzone", to whom there are a few references in Syberia (Amerzone cuckoo, etc). It's very short & very easy, but very atmospheric and memorable, like all Sokal adventure games.
The game really picks up again after Chapter 8.