There are some sites where you can monitor the water quality almost on a minute by minute basis.

Map

Tap water from the primary supply (streetside/at the meter) is probably safer than from inside any sort of establishment (home, office, hotel, etc.) because, for the most part, we pump water (up) into holding tanks, and then distribute it throughout the building. So the pump, holding tank, distribution pipes, even faucets can become contaminated quite easily.

Years ago the tap water here in Bangkok was pretty foul, both looking and smelling (sulphur). Now it seems pretty good, but again, highly dependent on location. The BMA is quite vast.

I don't drink tap water but have certainly swallowed some small amounts when brushing my teeth, no ill effects AFAIK.

Most Thais in my neighborhood get water delivered every week (and ice delivered daily), as do I (water). Some people cook with tap water, I don't.