The first adventure game I've had the desire to play for a long time and eventually beat.
I was drawn to the game while watching a college roommate play it. The environments were so large and detailed, and the fact the story was involved with the environment was super cool. Plus I like Persia/Arabia/Middle Eastern stories and history.
For better or worse, it was the easiest game to stay with. Take that to mean whatever you want. The fighting isn't that tough (especially the final boss, in which Ubisoft just seems to say, "Hey, congratulations on getting this far, here's something to occupy you for five minutes before the concluding cutscene."), but I don't think this game was necessarily designed for its battles.
If you've ever played a role playing game or less-intricate adventure game, you've probably reached a small fence or forest surrounding the playing area and said to yourself, "It's completely unrealistic that I can't just hop this fence or climb this tree." Well, in Sands of Time you can do that plus much, much more that is probably even more realistic but not as frustratingly limiting. The wall-running and puzzle-solving are by far the most compelling parts of the game. If you want a slash-and-run action game, look elsewhere.
Bottom line, Sands of Time was my favorite single-player game of all time. I seldom stick with single-player games; I lose interest unless I have a "teammate" to say, "Hey, want to play some _____?" Maybe it's the inclusion of Farah as a de facto second player that helps (and is lacking from the later two PoP's) the co-op feel. Maybe it's that whenever I got frustrated or bored with the game I could look at the beautiful scenery.