Maxvorstadt: I would love to buy Rollercoaster Tycoon 3 in this sale, but sadly only 1 and 2 are here.
RCT3 is at least as good as any game here IMO, so much to do and very polished. I hope it comes back soon. RCT1 and 2 are also among the best and have more to do than you might expect (compare with the recent commercial RCT2 clone Parkitect, also on GOG, that doesn't have as much to do even with the DLC and the few extra mechancs they added are just annoying IMO, although the main RCT2 game is still there). There is OpenRCT2 that is a reimplementation of the game engine for both 1 and 2 and supports modern resolutions (plus a scale factor to make the lowest zoom larger on high resolutions). It is a bit unpolished, particularly in the setup IIRC, but at least for me the game itself seems to work almost perfectly (except for the screen occasionally jumping randomly when looking around) and has for a few years (and is still in active development), although I haven't played through the full games in OpenRCT2.
https://openrct2.org Cities in Motion is another of my favorites; I'll mention that the base game has very little to do so considering the price difference of getting the actual content separately I can't recommend getting the base game alone. If you aren't sure if you will like it try OpenTTD; Cities in Motion is the same type of thing but with nicer graphics and focused on transportation within a city rather than between cities (well, same type of thing if you play OpenTTD like I do without competition). There are a few UI oddities in OpenTTD and documentation is on the wiki, so make sure you can check the wiki when you are learning (there are also tons of settings so if something in particular annoys you there is a good chance you can change it).
https://www.openttd.org/
I'll also plug Mini Metro (not at all a builder game), although the $5 sale price should be the normal price IMO and it isn't for everyone. For example there is no winning the game, you only try to not lose as long as you can. There are a number of challenges you can achieve, although the randomness makes some challenges unwinnable on occasion (IIRC, mostly the "don't use more than one bridge" challenges on maps with bendy rivers). Still, it is generally well balanced IMO and I find it fun to play a game or two at a time (maybe 15 mintues) or longer on occasion. The core gameplay is simple but there are more variations than you might expect. It would be a great tablet game (or sufficiently large screen phone) for people into such things (and is avilable).
Ok, one more, I'll mention Space Colony HD has some good and bad points; I paid a bit more than the current $2.50 price years ago and never made it past the first few levels before quitting in frustration, but I still don't regret the purchase. The bad IMO is that the challenge of the game is almost entirely (most levels I played) in preventing the different station residents from literally killing each other. You need to keep an eye on how much they dislike each other and tell them to work it out before they get too upset. This is not at all fun IMO and gets quite difficult as the number of residents increases. There is also some micromanaging of jobs that is meh. However, I like the building part and it has some unique aspects I haven't seen in other games (somewhat similar in Surviving Mars, but that game also has some annoying restrictions although overall works much better IMO).
