The Wood handbook has a rot resistance chart in the appendix. I am on mobile now to not going to download and search through that. If you can't find it, I'll look it up what I'm in front of my computer later.
www.fs.usda.gov
That is an engineering book, so hopefully architect will pay more attention to that than what "some guy on the internet" found. I'm not saying the opinions offered or wrong, just that if you're going to talk him out of poplar, a published book may help with it.
As others have said:
if you do locust make it black locust
If you do Oak, make it White Oak but make sure the sapwood is removed.
Cedar depends on what kind of cedar...
Redwood is a great option If you can get it.
Catalpa is much lighter weight but it still has good rot resistance. It was somewhat widely planted to grow fence posts in decades past.
The heartwood of longleaf pine is also very rot resistant... I spent a summer as an intern in the woods in coastal North Carolina, where we found property corners that were marked with heartwood stakes that were 100 years old.
American chestnut....we can wish for that! Maybe one day... I just collected and planted some nuts off of it population of "Restoration chestnut" (15/16 American).
