Michael has 37% more dialogue over the course of the series than the next highest speaker.
It makes sense that over the entire series that Michael Scott would have the most dialogue as the show focuses solidly on his antics as the boss of Dunder Mifflin.
Following him are Dwight, Jim and Pam, who are also a major focus of many story lines.
Recurring characters who have fewer lines include Erin, Oscar and Ryan. Other characters, like Darryl (1160), Phyllis (966) and Kelly (822), even though the were a present throughout the entire series, don't even break the top 10.
Dialogue by Season
Michael dominates the dialogue spoken for the first 7 seasons, but after he departs the show, first Andy and then Dwight take over as the top speakers for season 8 and 9. There is a bit of movement between the seasons for speakers outside the top 4 (Michael, Dwight, Jim, Pam), with Erin becoming a solid source of chatter once she joined the series in season 6.