Past or past perfect

They don't make any sense. Any reason for the relative clause?

seagullWhich sounds more natural to a native speaker?

Neither is natural.

seagullwhat is the most natural expression when we want to say things like this?

The man I (had) thought was my best friend deceived me.

seagullWhich sounds more natural to a native speaker?

I think those can be made 'natural' in a formal written piece with further elision:

a. The man I thought my best friend deceived me.
b. The man I had thought my best friend deceived me.

With the "who " clause, would this be a possibility?
The man, who I had thought was my friend, deceived me.

grammarfreakThe man, who I had thought was my friend, deceived me.

That seems to me to be awfully clunky. Gus's is by far the most expected.

I was just trying to fit the" who" in the OP into a possible sentence scenario. I agree, it is clunky! That said, sometimes these awkwardly written sentences certainly can be a potential mind field as I tried to walk through the answering process. I got in trouble in many times in the past..

grammarfreak a potential mind field

Time for the 2nd emoticon of my career!

Oooops!

Thank you very much, everyone.
I learned a lot from you.

Now I noticed I'd missed my earlier posting. I apologize for causing you trouble.

I meant to say,
a. The man who I thought was my best friend deceived me.
b. The man who I had thought was my best friend deceived me.

Are they still unnatural to a native speaker?

seagullAre they still unnatural to a native speaker?

No.