Dragons Lair had such shallow gameplay, plus lazer discs never really took off so not sure why people vote for it.
My vote is for Atic Atac The first true decent arcade adventure, a landmark game and quite fun to play. It set the mould for others like zelda to copy and improve.
Gyruss and Track and Field were also fun
I remember Atic Atac - I had C64 and friend of mine had ZX Spectrum, so we were hanging out a lot at each others home playing games that were only at their respective platforms. I can't say Atic Atac was one of my overall favourites on Speccy (unlike Stamper brothers year later game, Knight Lore), but I have very fond memory of it.
Also, great pick with Gyruss - most of my friends and me back then were into Scramble, Galaga, Pheonix and stuff like that, when it came to shooters, but one of them was really into tube shooters, both Tempest at first, and then really into Gyruss.