Where would be the best point of a rotating machinery , to take oil samples .

Location of the sampling point or points depend on what you are looking for.
Basically you need all the wear metals , contaminants etc. to be present in the sample.
similarly the sample should be taken with the equipment in use, if possible and obviously into a clean container.
If sampling points exist, make sure that any deadleg sample is discarded.

If the equipment has a fluid filter on it, install a sample port on the inlet (non filtered side) of the filter housing.

For the Particle count test ( how clean is our lub oil) Is it recommended to take a sample after the oil filter?
in reality I have not found any special recommendation for particle count test. However it seems to me that if we sample before the filter (as we do for wear detection) the information we will have will not be the be the real oil cleanliness in the bearing...the oil in the bearing is more clean and this could lead to changing the oil will the oil film is clean and the lubrication is good.

Please advise.

Many thanks.