Alex Ionescu
Windows kernel expert who started as a teenage developer of an open-source Windows clone and ended up co-writing the definitive doorstop on how the real thing works.
In the early 2000s Ionescu was one of the lead kernel developers on ReactOS, an attempt to build a binary-compatible Windows NT from scratch. That requires understanding the original better than most of its own developers do. The knowledge carried him to Windows Internals, which from the sixth edition in 2012 he co-wrote with Mark Russinovich and David Solomon.
He was one of CrowdStrike's earliest employees and has found and reported a long list of Windows kernel vulnerabilities. At conferences he is known for talks that walk through undocumented kernel machinery at a pace that has the audience photographing every slide, because no other source is going to explain it.