Best at being a DNS server?
Okay, not a problem for good number of users here.
If you want build a heavy-duty DNS server and want best OS for it. Gentoo is the choice.
How often do admins outside of datacenters and major ISP have to build a heavy-duty DNS server? Not very often at all.
Ivy Bridge i5-3570K@4.0Ghz, Gigabyte Z77X-UD3H, 2x4GiB of PC3-12800, Sapphire RX Vega 64, Corsair CX-600 and Fractal Refined R4 (W). Kentsfield Q6600@3Ghz, HD 4850 2x2GiB PC2-6400, Gigabyte EP45-DS4P, OCZ Modstream 700W, and PC-7B.