God that's weird. I was just about to start a thread asking the same question...
My top five, in no particular order, are:
1. Flying under the Golden Gate Bridge in F-18 Interceptor, and switching between the various out-of-cockpit views to take it all in. This was one of those moments when I sat back and thought "Wow, I'm glad I bought an Amiga".
2. Finally battling my way onto the roof of the cathedral in Batman only to see the Joker halfway up the ladder. Thinking I'd blown it, I launched the Batarang towards him in a fist-shaking moment of frustration... to see him fall off the ladder and plummet to his doom.
3. Doing battle with the giant splitting orb at the end of the third spaceship level in Turrican 2. I've yet to see this section bettered for sheer unadulterated arcade perfection. The 360 multi-layered parallax scrolling and the orb's treacherous splitting and slamming together makes for a relentless, dizzying and exhausting experience.
4. Trading insults instead of blows in Monkey Island. What a fantastic, original idea. There's so much humour and wit in Monkey Island and it's such an atmospheric game. This was one of the most enjoyable parts - trudging around the island looking for a fight to try out your newly learned insults in.
5. Two-player games of Lotus 2. Although hugely enjoyable on its own, having a mate around to race in Lotus 2 is always infinitely more enjoyable. We've often spent entire evenings ramming each other off the road or into trees while giggling like two 8 year olds.
