Certainty of modal-verbs

Suppose someone is knocking on my door,

My mom ask me: Who is it?

Me: That'll be Rihanna. / That would be Rihanna. / That should be Rihanna. / That is Rihanna.

Which is most certain, and which is least?

• Thx, I edit it. @snailplane – Kinzle B Mar 29 '14 at 10:41

Okay, we go one by one. I'm keeping the context in mind.

That'll be Rihanna - you are pretty sure (will implies sureness here - I will do this work - you are pretty sure doing it).
That would be Rihanna - Could be same as the above one BUT you also add a probability of that someone being Rihana here.
That should be Rihanna - You are expecting her and at this time, no one else but Rihanna is there. (She's very punctual!).
That is Rihanna -there's no doubt that she is Rihanna (say you are probably seeing her through the glass-door or CCTV!).

Out of given all, That is Rihana sounds the most certain way to tell that she is Rihanna and no one else. To me, the least is the sentence with would be. I feel that microscopically would/should/must etc. are your expectations, wishes or beliefs but when you say "is", it's the reality in such case. Check the example I gave there of the CCTV or a glass-door where you don't use anything but is Rihanna.

• What about That must be Rihanna? Any implicit situation? – Kinzle B Mar 29 '14 at 13:50
• @ZhanlongZheng an intensified version of that should be Rihanna. – Maulik V Mar 29 '14 at 13:53

The first two are equivalent. They express a certainty from the gut that the knock is from Rihanna. The third option expresses the possibility (however slight) that the knock isn't Rihanna.

And in response to the edit, the fourth option: "That is Rihanna" is the surest statement of all. Maybe Rihanna could be seen from a window?