Looking on the internet deeply has found these results:

which I believe to be. is the most popular phrase on the web.

who I believe to be. 12,420,000 results on the web	which I believe to be. 13,970,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
My wife Ron, who I believe you know. I have to think about who I am... and what I believe. I have to do what I believe to be right. People stand in front of me and lie every day, and I have to read between the lines and try to figure out who I believe. Better potential than any traditional pharmaceutical, I believe. You don't accept who I am or what I believe. I cannot unbelieve what I believe. You are exactly who I believe you to be.	It doesn't look like this phrase is very popular! The search could not find any examples on the internet.

who I believe to be.

12,420,000 results on the web

which I believe to be.

13,970,000 results on the web

More popular!

My wife Ron, who I believe you know.
I have to think about who I am... and what I believe.
I have to do what I believe to be right.
People stand in front of me and lie every day, and I have to read between the lines and try to figure out who I believe.
Better potential than any traditional pharmaceutical, I believe.
You don't accept who I am or what I believe.
I cannot unbelieve what I believe.
You are exactly who I believe you to be.

It doesn't look like this phrase is very popular! The search could not find any examples on the internet.