Brent Atchley was the earliest I can remember (February 2006)
The Shane O'Neill pro part that the Berrics charged everyone three dollars for (May 2010)
Dylan, obviously. This really set the groundwork for the media takeover that comes with a solo part, the skater's mentality of "I'm gonna do my own thing," and high production values for a single part. (August 2010)
Torey Pudwill's Big Bang - all of the above applies, but it was done through Thrasher and is a forerunner to the SOTY blitz that the award became.
Mark Suciu Cross Continental - to me, this was the last solo part to really have a massive impact, the way PJ Ladd's part did, before everything went all online. Suciu went from zero to hero with this one and I remember Thrasher even made a point to include a layout in a print issue dedicated to the part.