Tricky question @Trevor !
During the boot process, when "booting the kernel...." stage starts, a function start_kernel() is called. The execution of start_kernel() and the functions it subsequently calls occur with process ID 0, marking them as the ancestors of all kernel-mode processes as well as of the process ID 1.
So, this is the first process that runs during bootup ( PID 0 ).
Within start_kernel(), dozens of kernel subsystems are initialized, including the task scheduler, and the process eventually leads to the call to rest_init(). It spawns the very first user-space process by launching kernel_init(), which is assigned process ID 1 and is special because it becomes the direct or indirect ancestor of all user-space processes. Additionally, rest_init() creates the kthreadd process (usually bearing process ID 2), which functions as the parent of all kernel threads. The boot process reaches its final step when kernel_init() finishes, making way for the init process to take over.
Since you did not mention first user space process or kernel space process - hence the answer had to include the details.
"In an investigation, details matter..." - Jack Reacher
