How was the futuristic one? I've always been interested in this series, but I think the only one I ever played was the DS one.
For the 3D titles, I'd recommend 1404 as an entry-point for new players.
But you honestly can't go wrong with 2070 as a futuristic entry apart from the DRM Ubisoft cooked up at the time. It adds some solid QOL improvements plus new mechanics like: ecobalance, energy, the separate building factions, and air military units. Certainly has some of the longest production chains in the entire series once you go for fully-satisfied Eco/Tycoon Executives.
The consensus on 2205 is pretty mixed, I could write plenty about where it went most wrong and some right.
Off topic but in 1800 will they keep ideas from the last game? Some things were actually cool.
Off the top of my head, they're retaining the move building/object feature and (I think) modular system for building components/upgrades. It's based off the 2205 engine compared to the one powering 1701/1404/2070.
