I used phpBB and Drupal forums.

I used the Drupal 6.x Forum plugin. I don't think it was practical back then. I did not try the newest version of Drupal (8.x) but the problem you will run into with that CMS is when they move to version 9.x... it won't be 1 to 1 compatible and will require some work to switch to the new version.

As for phpBB, I think it was much better than Drupal because it was specialized as a forum platform. Easy to use as a user standpoint (I think that should be what you test the most to make sure users don't get turned off by the interface of the system you decide to use.) It has been a long time since I ran this one, so I can't really vouch for the latest version. But I think you are less likely to have problems when upgrades come out (compared to Drupal.)

As far as Google trust/ranking is concerned, I don't think either platform has an advantage or disadvantage. I would look at the URL and make sure they look clean. On Drupal, you could install a module to get the URL to look like the "Title" of the post (here it would be the question). By default, though, it's often something like example.com/forum/<number> -- that would be what would lower your ranking a bit. Search engines are not so good with numbers. Also, the path must be using dashes (-) and not underscores (_) in the path because Google recognize words separated by dashes. However, "something_like_this" is one word as far as Google is concerned. This will make a world of differences.

So to resume:

1. Make sure you have nice permalinks, with dashes
2. Make sure it's easy on the user so they post like crazy

I'm a huge SMF fan myself

Simple to install will supported great mods

Rick

vBulletin very good for me

Here are the list of PHP Forum Script/Software:

MyBB
phpBB
IP Board
Vanilla Forums
Simple Machines ForumbbPress
Phorum

This Image Show the difference between MyBB, XenForo and vBulletin

For PHP I'd suggest xenforo, I've been into forums a decade now and this is what I currently suggest to my clients.