This answer is not very colloquial. Another possible solution should be "Which tomato is the reddest?"

Or “Which tomato is the reddest one?”

"One" is acceptable... but kind of redundant. I would be more likely to say either "Which one is the reddest" or "Which is the reddest tomato."

Absolutely.

Why is there "itu" in the Indonesian sentence? Isn't it useless?