My favorite was actually Act 3. Act 2 was cool, but I was never a desert sort of guy. The dark and evil jungle just appealed to me a lot. I remember back in the day when my dad first bought the game for me and my brother. My older brother was the first to get to Act 3, and that was a big deal for us during the time because none of us had seen it yet. Also, Mephisto is my favorite prime evil and since he was the boss, it was more-so a reason to like it.
I remember the last bit of act 3 being really cool, as you come out of the awful swampy area and get to where all the temples are. Something mysterious about that place. Act 2 was alright though it did get boring after a while. Didn't enjoy the others as much