In a {Lambda} cold dark matter ({Lambda}CDM) cosmology, the baryonic Tully-Fisher relation (BTFR) is expected to show significant intrinsic scatter resulting from the mass-concentration relation of dark matter halos and the baryonic-to-halo mass ratio. We study the BTFR using a sample of 118 disk galaxies (spirals and irregulars) with data of the highest quality: extended HI rotation curves (tracing the outer velocity) and Spitzer photometry at 3.6{mu}m (tracing the stellar mass). Assuming that the stellar mass-to-light ratio ({Upsilon}_*_) is nearly constant at 3.6{mu}m, we find that the scatter, slope, and normalization of the BTFR systematically vary with the adopted {Upsilon}_*_. The observed scatter is minimized for {Upsilon}_*_>~0.5M_{Sun}_/L_{Sun}_, corresponding to nearly maximal disks in high-surface-brightness galaxies and BTFR slopes close to ~4. For any reasonable value of {Upsilon}_*_, the intrinsic scatter is ~0.1dex, below general {Lambda}CDM expectations. The residuals show no correlations with galaxy structural parameters (radius or surface brightness), contrary to the predictions from some semi-analytic models of galaxy formation. These are fundamental issues for {Lambda}CDM cosmology.