Don't forget that the internal batteries on these won't last 20+ years. In fact, they won't last 5+ years. Most of them are prone to "swelling" - where the breakdown on the internal chemicals of the lithium batteries break down and create hydrogen gas. This can be a safety issue - although spontaneous ignition is highly unlikely. But it can also be a cosmetic one - the swelling of the cell could cause the shape of the phone to change - like the whole backplate of the phone to come off. And then, lastly, the phone won't work in 20+ years because the battery will definitely be dead. And while there's some value to something just because it is what it is, the fact that it doesn't work will reduce the value.
If I had to say which phone I think would be worth a lot, my guess would be an original iPhone 1 in the original box with the wrapping still on it. But if you don't have one of those, then I don't know that I would recommend anything.