Sick's tried-and-true form factor has an IP69K housing, and every possible sensing style. It is available with a Teflon coating for chemically harsh environments.
The INOX family is an IP69K, ECOLAB-rated, stainless steel sensor. It mounts easily almost anywhere, and is available with the full spectrum of sensing options.
These sensors are completely enclosed in stainless steel, with no mounting holes or slots that could accumulate product. The mounting system is designed to be completely smooth. The teach-in is via a stainless steel membrane.