air quality

The air quality in Beijing is better than before but probably is not up to what the world is/would/might expect.

In my view "would" and "might" are okay but not "is". You cannot say "... not up to what the world is expect". Without "is" it is fine. ".... what the world expect."

Sorry. I wanted to say "the world expects"

Of the three, which is better? Why?

The air quality in Beijing is better than before but probably is not up to what the world is/would/might expect.

I would say would.

Although, I would word the sentence differently.

The air quality in Bejing has improved, however, it is still not up to what the world is expecting.

You said you would say 'would'

But your improvised version uses 'is' expecting.

Could you help me understand what you're trying to say?

Thanks.