Which one is correct-Ali is taller than you or Ali is taller than you?

What do you mean? that if it should be a question or an answer? well we could say both, you could say that "Ali is thaller than you/him/her" and you could ask "Ali is taller than you/him/her?" but the formal and correct way for this question to be asked would be "Is Ali taller than you/him/her?" that would be the formal way to say it.

Not sure about this next part but on a situation where the person asking the question is on a shock/surprised state, or verifying something already said, maybe the correct way would be "Ali is taller than you/him/her!!!?"

Scene 1:

You: I found out Ali is taller than me

Jean: Ali is taller than you? ( Surprised or Verifying what was prviously said )

You: Yeah, i though we were the same height, or that i was a little taller than Ali.

Scene 2:

Jean: Hey i wanted to ask you, Is Ali taller than you?

You: Yeah, He/She is taller than me.

Jean: Oh, just making sure, it didn't seemed tha way.