I have individual bedrooms as soon as possible, but what makes my fort a fort is a well. I love having one in the middle of the fort, with a hospital to one side, and a soap stockpile and a bucket stockpile. It is usually aquifer-fed, always at least 4z deep, and I have been known to make a waterfall entrance. I train up a mechanic so that I can have masterwork mechanisms, I buy fancy decorated wooden buckets and expensive dyed and decorated ropes to make it with from the merchant wagons. I make heavy gold blocks and fancy rock salt blocks to make them with. I tile the floor and engrave the walls. I put gratings around it and statuary.
My nobles get a "nobles only" wellroom next to the nobles quarters, and I will make lesser wells into the same cistern with chains next to them, so my prisoners don't die of thirst when they're incarcerated. Sometimes, late in fort development, but early in planning, I'll put in a third well in a more extensive hospital level, complete with private rooms around the edge of the wellroom to wall off in case of were-transformations. At other times, I'll make a "dwarfchild play bunker" with a food storage area, a few tables and chairs, a wellroom, the kids bedrooms, and a toy stockpile. It can have a small locked room with a pick stockpile too, in case the kids grow up and have to dig their way out. I can lock it off from the rest of the fort and the kids survive whatever flaming forgotten beast is nearby.
My forts might lack other things, but there's always a well, and usually a waterfall.