If I'm being serious, I would say the computer.
Also the person who invented air conditioning and central cooling/heating should forever have their balls licked by the world's most beautiful and orally talented people. You wouldn't just happen to be the inventor of such a wonderful device, are you Nick8? :biggrin:
The problem is.... the computer wasn't invented in the 20th century. Charles Babbage, who died in the 1800s, was the father of the programable computer.
Air conditioning has a better, but not entirely exclusive claim. The AC you know and love was invented by someone you've heard of: Willis Carrier, whose company still bears his name. Air cooling and compressing of gas to chill air, however, has been around since the 1800s. Ancient Egyptians in the times of the pharaohs would take advantage of the Peltier effect of extreme desert temperature changes and create sheets of ice which they made from moulds laid in the ground, hanging the ice in the window of the prevailing wind to cool the house. This technique and others were used to provide central cooling and heating in wealthier homes. The Forbidden Palace had a rather sophisticated setup of cooling with misted water and fans in some rooms, just like modern swamp coolers. so I'm not sure if air conditioning counts as a 20th century invention.
Which brings me back to antibiotics. I did some looking around and antibiotics have been used since forever though people didn't know why they worked. Some plants, like sundews, were applied to wounds because they sped healing and reduced infection because of their natural antibiotic properties. The physicians in Egypt during the Islamic/Ottoman Empire used to mist wounds with alcohol and keep patients isolated as they noticed infections went down when they did so. Nobody seems to have figured out the cause of the infections, but they did occasionally stumble upon ways to mitigate it. So again, I'm not sure antibiotics count as a 20th century invention.