SimCity 4
Rollercoaster Tycoon 2
Not only are those my 'go-to' games, I never even left them in the first place. That's the beauty of endless games. You can only play a story-based game so many times, but endless games they... never end.
There's actually a third, but since a year or two I 'permanently' switched from the older game to the newer:
Anno 1602 > Anno 1404
Then of course there's the series I always buy a new game from, since like forever, like Super Mario and Zelda. I wouldn't count merely replaying a certain game twice every decade a real 'go-to' game, otherwise I'd have to fill the list with a whole bunch more games.