Whom to suggest as an editor when submitting a paper? Some journals let authors to suggest who should handle their submission. How should one choose the suggested editors? In my particular case, I struggled with selection because


One of the possible choices was an editor who was one of my co-author's opponent when his dissertation was publicly examined
One editor was affiliated with the same institution as one of my co-authors
One editor had published with one of my co-authors
Two editors were well known by my co-authors (no shared projects)
Two editors I had never heard about


Our paper had about 10 authors from 6 different institutions. I selected an unknown editor because I felt there was some kind of potential conflict of interest (even if just perceived) in selecting someone else. However, I have been wondering whether I did the correct choice? My paper got eventually rejected but I got some nice feedback to improve the work. Therefore, I might face the same situation soon again, when I get to the point of resubmitting the paper to some other journal.
You are correct, selecting an editor the authors have not met avoids perceived conflict of interest, which is a good thing.  I think "editors were well known by my co-authors (no shared projects)" is also a good choice.

It's not always possible to eliminate all potential conflicts of interest.  I would prioritize avoiding selecting an editor whom I had coauthored a paper with, since that is a very important form of professional relationship.  Ultimately, assigning the manuscript to an appropriate editor should be the journal editor-in-chief's responsibility, not the author's.