I was watching the Bayonetta developer commentary a little while ago and realised that, for me, Bayonetta would be one of the games that defined this generation of gaming.
It's not my favourite game of this generation, but I think that it really represents many of the defining characteristics of current gaming in my view.
So, what of previous generations?
8-bit? I'd say Super Mario Bros on consoles and Boulderdash on home computers
16-bit? Street Fighter on consoles and Populous or Doom on home computers
CD-era? Final Fantasy 7 (for better or worse) on consoles and Myst on home computers
DVD-era? Shadow of the Colossus (games moving into more meaningful territory) or Halo on consoles and StarCraft on home computers
Current?.. it's not over yet :)
Not trying to make a definitive list or a list of favourites but rather one that highlights defining titles that made an indelible impression on gamers and market forces as the hobby evolved. I'd be interested to hear of the views of others - if only to re-assess titles I may have been dismissive of at the time.