Its been said that your kitchen counter is way dirtier than your toilet seat. That’s why I spray Lysol or Fantastic in my sink after every dinner.
I voted for bath tubs because I feel like mold, mildew, and soap scum are the biggest factors affecting cleanliness. Sinks can get dirty with toothpaste and general hand dirt, but the tub seems like a more daunting task.
A good question to ask next is, how does the material of the sink / tub affect its cleanliness? Think water - hydrophilic vs hydrophobic.