CC0
The public domain dedication, and the freest "license" there is.
CC0 1.0 is Creative Commons' public domain dedication: the creator waives all copyright and related rights worldwide, as far as the law allows. Unlike "royalty-free" (which still comes with a license agreement) or CC BY (which requires attribution), CC0 attaches no conditions at all. Commercial use, modification and redistribution are allowed, and no credit is needed. Well-known asset libraries such as Poly Haven, ambientCG and Kenney use it for the same reason: it takes license questions out of your project. On Grab3D, every model is released under CC0 (all download formats, the lighter variants and the rigged versions included), so you can ship it in a commercial game without tracking licenses or credits.