I think it would be the Kitchen/Diningroom/office.
I'd like to knock out a wall and put in one of those dividing cupboard/counters. Knock out a window and put in patio doors. Close off a door and put in more counters. New fridge, new stove, new cupboards, new flooring, new lighting. Hmm... I wonder if $25,000 would be enough. I think not.