A quantum emitter efficiently coupled to a nanophotonic waveguide constitutes a promising system for the realization of single-photon transistors, quantum-logic gates based on giant single-photon nonlinearities, and high bit-rate deterministic single-photon sources. The key figure of merit for such devices is the \(\beta\)-factor, which is the probability for an emitted single photon to be channeled into a desired waveguide mode. We report on the experimental achievement of \(\beta = 98.43 \pm 0.04\%\) for a quantum dot coupled to a photonic-crystal waveguide, corresponding to a single-emitter cooperativity of \(\eta = 62.7 \pm 1.5\). This constitutes a nearly ideal photon-matter interface where the quantum dot acts effectively as a 1D "artificial" atom, since it interacts almost exclusively with just a single propagating optical mode. The \(\beta\)-factor is found to be remarkably robust to variations in position and emission wavelength of the quantum dots. Our work demonstrates the extraordinary potential of photonic-crystal waveguides for highly efficient single-photon generation and on-chip photon-photon interaction.