That's a difficult one, I've played so many over the years. I don't think I could pick just one.
I suppose an early favourite was the original 'Prince of Persia' on Amiga. That's the only game I ever bothered to speed-run. It had a time limit of 1 hour that I managed to get down to about ten minutes approximately. That said, I have much more play-time logged on Monster Hunter World, or No Man's Sky, both of which are strong contenders for a favourite. I'm also a big fan of the Dynasty Warriors games. Oh, and Horizon: Zero Dawn was great!
See? I just can't pick one at all! So annoying.
If you were on death row, what would you pick for your last meal?