Do you use articles in these cases? If you use, which one?

1. Which is bigger, cat or mouse? (In general)
2. Which is cheaper, coffee or tea? (At a specific cafe.)
3. Which is cheaper, bus or train? (In general)
4. Which is more tasty, pear or apple? (In your opinion, not about a specific pear or apple)

See a translation

I would use:
1. "Which is bigger, a cat or a mouse?" (or Which are bigger, cats or mice?)
2. "Which is cheaper, the coffee or the tea?" (If you are asking at a specific cafe.)-- But, if you are asking a question in general, you would say "Which is cheaper, coffee or tea?")
3. "Which is cheaper, a bus or a train?" (I would use either "a" or "the" as in "the bus or the train.")
4. "Which do you like more, pears or apples?" is how I would ask this. or I would say "Which do you think are more tasty, pears or apples?" (so it's clear I'm asking their opinion).
You could also ask "Which do you think is more tasty, a pear or an apple?"