According to the National Weather Service, the lowest sea-level barometer reading was 28.65", recorded on Dec. 12, 1995. That was during a fairly large windstorm that looked like it was going to do considerable more damage than it actually did. Despite the strength of that storm, it moved north just enough to keep the damage from becoming extensive.
On the flip-side, the highest reading was 30.83", recorded on Dec. 3. 1921 at the Federal Building in downtown Seattle. We don't have the specific weather of the day, but it was likely due to a strong arctic air mass overhead, which usually produce very high pressures.