Your bathroom sink.
Your bathroom sink believe it or not will always be the most smelly Mc Nasty sink in the house.
You spit your tooth paste in there. Tooth paste has a fine sand base, this base coats and holds to the sides of the piping and breeds bacteria which smell bad. Mix this with your morning dose of morning mouth wash out and yuck city.
If you can. take out the stopper and clean it, you may have to disconnect the lever. You may have a nasty old bottle brush to put in there and flush the gunk out like I do, or you may want to buy one, they're cheap, cheaper than me.
BTW, drain cleaner always makes the drain smell worse.
If you have a good wet dry vac **** out the drain. Stinky, yes.
Put in some odorless bleach to kill all of the bugs. NOT IN A DRAIN WITH DRAIN CLEANER IN IT! thank you.
You may need to do this weekly.
Or, after you do a rinse with the mouth wash. dash a spot at the drain.
A good habit is to run the drain for 15 to 30 seconds after you are done brushing flossing and spittling into the sink, this flushes all of that microbial smell making bacteria down the drain.
Most people who do a spit and or brush and do not flush the junk down the drain see me a lot more than those who follow my advice. They think they are saving water. If 30 seconds of rinse out is too much water that's just silly. But if they want to save the world that's OK, it keeps us busy.