It's usually the trap(hopefully).
Takes about five minutes to drop and clean. Buckets and towels.
Snake the wall from there.
Use hot water and fill the sink/tub/toilet until there is a good seal and vacuum.
The plunger works best when pulling up.
Slow pull then shake shake shake.
Use an enzyme for preventative maintenance like
Bioredex Natural Drain Cleaners - The Direct Source for Bio-Cleanand MegaMicrobes
I use 1-4 cups white vinegar occasionally in the dishwasher to get rid of hardened soap and calcium.