The type-II Weyl semimetal ${\mathrm{WTe}}_2$ is a key material for exploring emergent quantum phenomena; therefore, there is intense demand for high-quality single crystals. Here, we report the successful growth of ultra-high-quality ${\mathrm{WTe}}_2$ single crystals using an improved Te flux method, achieving a world-record residual resistivity ratio (RRR) reaching $\sim 4000$, nearly doubling the previous record. These crystals exhibit an extremely large magnetoresistance of $\sim 1.3\times {10}^7\%$ and an average carrier mobility of $4.1\times {10}^5\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at 2 K and 9 T, both surpassing previous records. By systematically comparing crystals spanning a wide range of RRR values, we demonstrate that crystal quality strongly influences low-temperature quantum transport and Fermi surface (FS) properties in ${\mathrm{WTe}}_2$. The exceptional quality and size of our crystals enabled magnetic torque measurements and the first observation of de Haas-van Alphen (dHvA) quantum oscillations in ${\mathrm{WTe}}_2$. Furthermore, combining Shubnikov-de Haas and dHvA oscillations allows for a reliable three-dimensional mapping of the FS below 9 T. Through an angular-dependent analysis using both probes, we determine the complete set of FS parameters over the full angular range. The nearly constant and very small Dingle temperature $(1\sim 2 \mathrm{K})$ indicates long quantum lifetimes and weak, nearly isotropic scattering, highlighting the intrinsic electronic character of the ultra-high-quality ${\mathrm{WTe}}_2$ crystals. Our work demonstrates the critical role of crystal quality in unveiling the intrinsic electronic states of quantum materials and provides a general strategy for improving the quality of other telluride crystals, enabling further advances in quantum materials research.

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• Magnetotransport
• Dichalcogenides
• Single crystal materials
• Weyl semimetal
• Crystal growth
• Quantum oscillation techniques