Hottest in terms of the destination, and fastest in terms of the speed that the space craft will zip through.
Article: https://motherboard.vice.com/en_us/article/humanity-will-touch-the-sun-with-the-fastest-spacecraft-ever-made
NASA mission website:http://solarprobe.jhuapl.edu
It sounds so kerbal. 7 Venus fly bys, getting to within 5.9 million km at closet approach to the sun, zipping at ~200km/s.
P.S: they call their data download speed "science downlink rate"