Personally, I like really, *really* hard games. I am several different types of masochist.

Fortress Defense is pretty dang good. For reasons I don't particularly understand all the FD races I've encountered in my most recent world are peaceful to my civ, but with some proper dwarfy torture I think I can fix that.
I rely super heavily on traps. I find having a huge hallway of ballistas and spiked balls and spike traps on repeaters to be incredibly satisfying.
My most recent project has been trying to figure out how to automate flinging weaponized sieger corpses back at their friends. Progress is slow but fun.
I also frequently make my own ludicrously hard enemy civs to fight (like "dragons" with steel armor and a nasty attitude that come in huge waves) just to kinda set my own doomsday alarm clock. Eventually they'll just come and vaporize me and I need to start over (or just use a drawbridge and wait for the spikes to *eventually* take them out).
question that I just randomly thought of: Since now there's an invader cap in the init, does this mean that there was originally no limit (which I doubt) or that we can now modify the limit that already existed? If the second one's true that could make some fun zerg rushes on your insane computer over there.
also just dig really deep. there's fun stuff down there. I've never actually gotten to the HFS, even though I've been obsessively playing this game for like 5 years now. My computer sucks and by the time I get down there my FPS is sad and I start over. Now I mostly stick to tiny forts close to the surface and rely on goblinite.
I want to make a super aggressive unicorn civ now. that would be hilarious.