Currently Online 
It's pretty hard to pick moments since it's usually descriptions of worlds and places that stand out for me.
There is one chase sequence in Tad William's Otherland that had my heart nearly beating out of my chest. It's near the beginning on the book where the main character is being chased in cyberspace. I was sitting in the tram with my stop coming up, trying to read faster and faster hoping she would be save before I had to get up and get out of the tram.
Best sci-fi
1. Otherland, Tad Williams
best moments: Entering each new simulation
2. Rama series, Arthur C. Clarke
best moments: Exploring the ship and inhabitants
3. Foundation series, Isaac Asimov
best moments: Explanation and application of psychohistory
4. Time Oddysey series, Arthur C. Clarke / Stephen Baxter
best moments: Surviving the sunstorm / Battle between Alexander the Great and Atilla the hun
5. Ringworld series, Larry Niven
best moments: Crash landing on the ring world / Escaping the ringworld through a mountain top
