What causes foul smelly saliva ; have excellent dental hygiene but saliva is very stinky?

Mahybe hygiene. Not knowing what you are doing for "excellent hygiene" makes answering more difficult. A mouth rinse and toothpaste called closys are formulated to neutralize the volatile sulfur compounds that cause breath problems.
Bad breath. If gum disease is ruled out, it can be from chronic sinusitis and/or acid reflux. Depending on what other symptoms you have, such as nasal congestion, post nasal drip, sore throat or stomach pain, the next steps in treatment differ.
Bacteria. Usually smell can come from bacteria. It could be in your saliva or in between teeth or in your gums or in back of your throat. Other possibilities are reflux and other pathology's. You could ask your dentist or your ENT to see possible reasons.
Many things. I'm not sure one can attribute a foul smell in mouth specifically to saliva itself. Anything going on in your mouth, sinus, nasal cavity, throat, stomach etc. Can present itself as an unpleasant odor in your mouth. There are many dental as well as medical causes. See the following website and then consult with your dentist first, followed by ENT if necessary-http://smilesapartcosmeticdentist.Com.