In the last few years I've spent a ridiculous amount of time sandboxing on RCT2, or its open source equivalent, so it definitely gets my vote.
It's amazing that it is still growing as a game after all this time and there are some wonderful things you can do with it now.
To me it's the perfect place to throw any creativity I've got in at a reasonable pace, due to how much I've gotten used to the system and how relatively easy it is to make things.
I play it almost theraputically, like some people and their adult colouring books. It's an art form and a great way to relax just knocking out dream theme parks.
