The kitchen. We had a builder in removing an internal wall and sorting out power, I'd already removed the old floor tiles and adhesive last week. I started on Saturday ripping out most of the old one, then running new hot & cold water pipes with a more sensible routing in the new room layout, with connection points for the dishwasher and washing machine in the right place. I did my first "proper" plumbing using soldered connections too - something I've been meaning to try for ages, and as MrsH has a torch anyway (she used to use it for her glass making before upgrading to one that takes both propane and oxygen) it was cheap to have a go and is very easy.
Then putting in cabinets, modifying where needed to go around the protrusions into the room from service ducts (soil pipe etc.), redoing waste waster all the way to the outside (first time using solvent weld rather than compression - very easy too) and finally temporarily refitting the old worktop, cut up appropriately so that we have a working sink and hob until the real one gets made after christmas.
Probably not the most sensible time to rip out the kitchen, but it did give a very firm deadline for having it work again, which we managed. Now there's just the ceiling to replaster (which my builder will do once he's back off holiday), finish the high cupboards (which I've left to give him clearance to the ceiling) and putting down a new floor.