Hard to say... I think the 3 most influential titles ever released are Pong for the Atari 2600, SMB for the NES and DOOM on PC. Yes I know that none of them were the first of their kind ever made, but they remained arguably the most iconic. For a lesser extant we could include titles like Tetris for the Gameboy, World of Warcraft for PC, Half-Life/CS for PC, Halo for Xbox and FF7 for the PS and Mario 64 for the N64.
Choosing one... that is one hard task. But if it wort's anything, in my opinion the greatest game of all time is Super Mario Brothers. Still totally playable to this day and it made the industry as failproof as it is at the current moment.
Now for my personal favorites, I would consider far less relevant titles, but they were truly exceptional in my eyes:
God of War for the PS2 , Resident Evil 3/4 for PS and PS2, Rayman 2 on PS, Warcraft 3 on PC, Rome Total War on PC and Red Faction on PC, Crash Bandicoot 3 on the PS, Quake 2 on PC, The Last of Us on PS3, Red Faction 2 on PC. Great titles IMO.
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