What's the fastest way to digitize contour lines?

Vector files: turn on Select Entity, click any contour line — the entire polyline is captured in one click. Raster files: try Easy Trace (drag the mouse along the line with the left button held) or Auto Trace (click three points: start of line, ~1″ along the line to set direction, then end). Define the background color first via Raster/PDF → Background Color. Auto Trace can struggle when many lines intersect; if it does, fall back to manual digitizing.