You can pick anywhere and honestly it's fine. Lots of people started with modern Who, so that lines up with the average experience, and there are long-running plot arcs you might stumble over if you don't, but most episodes are fairly self-contained.

Old Who is a different show. Somewhat akin to the original Star Trek, it requires a certain ability to ignore the experience and budget constraints and pick out the charm. But some of the old Doctors remain fan favorites like Tom Baker, the 4th and longest-running generation.