BUMP! sorry, I couldn't let this thread lie - I had to get my two cents in.
I'd nominate for consideration..
Knight Lore (Most UK 8bit machines especially ZXSpectrum) - when this came out everyone was gob-smacked - every second game for about 3 years after was a clone of it - even today it looks really smart - and this was written in 1983! (released '84) - and as well as introducing a new genre, really upped the ante for developers to produce quality games.
Virus/Zarch (Amiga/Archie etc)- this made me buy my amiga! Nothing like it seen before - colourful shaded vector graphics - a control system that needed mastering, and a real sense of fear and panic. Again gave the old 3D wireframe/polygon genre a huge boost.
Pinball Dreams and sequels - apparently sold over a million worldwide on Amiga (Note to self... must check accuaracy of this statement.) Must be one of the best sellers.
Tomb Raider - love it or hate it, and especially hate the hype, but when this was just another PS/Saturn release many years ago - it hit people like a ton of bricks. Lots of old fashioned gameplay elements moulded into a fantastic looking game. Later hype and endless sequels can make people forget how wonderful it was the first time they played this.
Oh and must second FFVII for 7-Zark-7 - my feelings for this game can't be put into words.
These would be at the top of my list, anyway.