The challenge is asking what other poster's flattest routes are?
Well consider your challenge completed because you succeeded! Good work!
All my rides upload to Strava at which time Strava decides to change the elevation gain to whatever they think it is. Guessing how much elevation Strava will remove from my GPS recorded ride is a game I now play after each ride.
Strava shows a 36mi ride I did a couple weeks ago as having 285' of climb which is under 8'/mile. My GPS shows it was 1332' of climb. Go figure.
There is a 44mi out and back thats all paved path on an old rail trail which is 44mi and 512' climb which ends up being 11.63' and I am pretty sure that is actually accurate. Its a 1% grade for half the ride up and 1% grade for half the ride down, going both ways. The only 'climb' is an approach to a road where the trail used to go in a small tunnel under the road, but the trail now goes up and over the road. Its a whopping 15' high, maybe.