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.