Utopia - Creation of a nation. There was never a game that recreated the feel of colonization of an alien planet as well as this one.
Dune - One of the greatest adventure games of all time. I actually played in on an Amiga emulator a couple years ago and finally finished it.
Syndicate - I still can't believe how this game wasn't remade yet. There were a couple of very poor spinoffs, but nothing even remotely close to the original.
As for a but more modern games, I'd love to see a remake of Elder Scrolls IV Oblivion, like the Skyrim Special Edition. I suspect Oblivion and Skyrim were the last great Elder Scroll games, before the era of microtransactions / passes / games sold in many little pieces took over.