the original half life. that game blew me away, and it created two mods that IMO are also two of the greatest games ever made. one being CS, and the other Day of defeat. day of defeat used to be absolutely incredible before valve took over and mainstreamed the crap out of it. 2.0 and before, there was nothing like it.
some others for me are earthbound wayy back on the SNES. final fantasy 7 for psx, so many good ones...
