Morrowind, in the sense that I ended up sightseeing more than playing. I was really in awe of the skies, the waters, the weather, and the general landscapes. I loved my virtual stay in Vvardenfell.
Maybe other games have increased the eye candy since, but Morrowind's pacing, design and novelty played a big role. To a much lesser extent, Far Cry was also quite betwitching, especially with its freedom to dive into clear blue water from lush bright green cliffs.
Apart from that, Symphony is quite mesmerising too, especially as your selected music could contribute to the transe. I often find myself "dancing" with the enemies more than maximising my fire power (without realising it, I end up moving and fire in tempo, which is a disadvantadge).
Some management and strategy games of all sorts can end up mesmerizing you with their little AI characters going through their acrtivities. Tropico, Evil Genius, etc. Basicaly, all games that has you set up a system and watch it function can do this. Even some tower defenses.
And then, there's these madmen from Amanita Design.
Seriously...