I've already caught a bunch of flak over this in the PS3 vs 360 thread, but it fits the topic of the thread:
Halo: Combat Evolved
Not because it's necessarily a bad game, far from it, but because I truly believe that without it the original Xbox would have failed. And you're going to have a Sisyphean task trying to convince me that Microsoft, of all companies, entering the console market was an overall net-positive for the industry. Normies be damned. Microsoft has bought-out, and sequently shuttered, literally dozens of developer studios over the past 20+ years. Which, part of the second-order effects of this contributed to the hyper-inflation of the production costs of AA & AAA games over the years... and I hate it..
Yes, I know that Sega was already circling the drain, but at least the Dreamcast would've had a longer, more natural lifespan..
I also believe that there would've been a greater incentive for developers to master the PS3's Cell Processor architecture when it finally came around as well..