Definitely "fire prevention system utility panel". It's the first first thing all my Sims households will get.
I like cooking on Sims 4 and I won't let anyone cook anything else but salads or BLT without it. I learned it when my Sim died in a fire that started when he was cooking. I tried to make him run to shower but he refused. He also refused to run away from the stove or extinguish the fire. I had forgotten to save the game, so I couldn't exit the game without losing everything (it was a new save).
Other favorite objects are the kitchen counters and appliances that came with Jungle Adventure. Also the Wabbit tablet for toddlers, it's a great way to keep them busy and learning skills when I want to concentrate on other Sims, especially on 100 baby challenge.