I tend to drift towards city builders that blend general design with system building and problem solving.
Lately I've really been into traffic simulation and transit design with systems that focus on relatively realistic cities, but I did really enjoy what I considered "Tool-chain" games when I was younger, such as Caesar III, and more recently the Anno series.
I'm really enjoying most parts of Cities: Skylines, but the traffic design tools leave a little to be desired. (Weird random 'snap' placement and curves at times, lack of pre-planning that leads to frustrating budget issues.)
But what I really want is a good solid and realistic colony game. Outpost 1 revamped kind of thing. There have been a number of 'colony' games that I've seen lately, but they're all horribly shallow and short tool-chain builders.