I’ve got two contentious ones (for different reasons), but I stand by them. They hold 2 of my 3 top all-time "played’ positions despite having gotten them recently in life (the third is Dominion, which misses this list by just a few centimeters)
Game 1? Spirit Island. I think it’s the best cooperative survival strategy ever made. The “almost” in its perfection is the need to ease a new player in, but my experience is that literally anyone can “click” with a low-complexity spirit and run with it indefinitely. And then the game has room to grow in every direction. Complexity? More/different spirits. Difficulty? The base difficulty leads you to nearly 100% win rate, but adding events or adversaries give a slow (but real) increase up to the highest difficulties that approach a 0% win rate even at the highest level of play. Why is this contentious? It’s popular and new, and everyone (including me) is hesitant to pick a game like that as a GOAT.
Now the opposite side of the coin, possibly edging out Spirit Island slightly… Kingdom Death: Monster. Let’s get the “not quite perfect” out of the way fast - the price is absolutely oppressive, and buying my used copy was as much of a risk as an investment. Knowing the current expansion list is over a grand and there’s another expansion set sitting in the store at $1250, that’s a hard pill to swallow if you need to own everything about a game! But you talked theme in Terraforming Mars - KD:M is the king of theme. It’s a brutalist masterpiece that has no hesitation turning a mistake into a “time to start from scratch” campaign-wipe at the 30 or 40 hour mark. Its mechanics are reasonable enough for any gamer to pick up, but its playstyle is simple enough that even a non-gamer can become fully immersed as long as they have someone running the rules in the background for them.