If you told me I could only play one game for the rest of my life, I'd choose Kerbal Space Program.
Althought at time of posting my steam time with this game reads as 99 hours, it's much closer to 600+ as I owned the non-steam version since 2013.
The game is adorable yet brick hard. Frustrating and immensly rewarding all in the best possible ways.
The Kerbals although never really characterised somehow have bundles of personality. There is a child-like naivety to them and their fearless quest to get to space on some very questionable equipment.
The game demands precision and has a steep learning curve, yet somehow there is a fly by the seat of your pants approach to everything. The parts have comedy descriptions throughout "A flatter variant of the OKTO, OKTO2 was most certainly not conceived when one of our engineers sat on a standard OKTO."
There is hours of fun in just building rockets and space planes and flying them before you even manage to achieve orbit, nevermind land on the Mun(Moon) and Duna(Mars) and beyond.
The fun doesn't stop there with a vibrant community of modders who have made changes as small as incresing the strength of joints between parts, to adding new parts for even more varied building, to adding in whole other objectives/game mechanics and even changing the solar system itself to reflect real life size of planets.
I highly recommend the game, and would also recommend any new players and intermediates who are finding difficulty progressing to look up a tutorial series on youtube by a man called Scott Manely, who covers everything from your first 3 part rocket to landing on the Mun.