Should it be 'were'?

One was ...

Whoa, déjà vu.

Yes, it should be. Two buses were ferrying passengers and one of them hit the kerb.

But the noun of the sentence is "one" as in one bus was ....

Either is correct.

One of the two buses which was ferrying a group of tourists.... (the verb refers to the one)

One of the two buses which were ferrying a group of tourists... (the verb refers to the two).

Incorrect.
There is only one subject of the sentence. According to grammar rules, the verb must agree with that subject.

The subject is singular—so the verb must be as well.

The subject is ‘one’; the verb is ‘was.’

Ramalamadingdong.

I think you are wrong...

In the second sentence, "which were carrying passengers" is a subordinate clause describing the two busses.

Cersei was one of the characters who were fighting each other for the iron throne...

In this was "which was fighting each other" doesn't make sense .

Do you know the subject of the sentence you just wrote?

Cersei?

Who said a subordinate clause has to be about the subject?

Yeah. So you changed the sentence construction, and now the question that was asked by the OP isn’t applicable.

When the sentence construction is

One of the Lannisters
Each of the students

the subject of the sentence is the singular one and each, but it sounds wrong to our ear to use a singular verb directly behind a plural object of a preposition.

But you have to anyway,

‘cause the s/v rule...

You are inventing a rule of grammar that doesn't exist. There is no rule in the English language that says that a subordinate clause has to describe the subject of the sentence.

One of the Lannisters who were fighting each other to see who would lead the family got stabbed in the back.

The subject of the sentence is "One". The verb in the subordinate clause is "were". It is plural because the subordinate clause refers to the Lannisters (as a group).

There are countless examples where the verb of a subordinate clause doesn't refer to the subject of the sentence (or agree in number with it).

Going back to the original sentence.

How many buses hit the curb? Maybe that will clear it up for you.

Your problem is that you don't understand what a subordinate clause is.

One bus hit the curb, yes. That is why the first sentence is correct.

Two buses were ferrying passengers. That is why the second sentence is also correct. In the second sentence, the verb "were" is part of a subordinate clause attached to the two buses.

They are both correct. Does that clear it up for you?

I have given you several examples to show you how you are wrong..

One of the two buses which was traveling together on a school trip hit a road kerb, and crashed into a car.

Does this sentence fit your rule?

Haha. Very funny.

"Was or were" revisited.

Listen, slick, the subject is indeed one, and verb is hit, while the object is the kerb. The subject is modified by the adjectival phrase of the two buses which were ferrying passengers. Within the adjectival phrase, the simple past of the verb to be must agree in number with subject of of the phrase, which is plural, and therefore, one must use were. Miss Tan now has a better command of the language than 95% of the members at this site. She was right from the outset.

I want to point out that Sentanta is agreeing with me.

Any time that Sentanta is agreeing with me, you can be confident that he is correct.