So let's start here - he's #12 all-time in Appx Value (equivalent of WAR), and #6 all-time among QBs, tied with Dan Marino and just ahead of John Elway.
If we're looking at QB Rating, Rivers is #4 among those who have completed their career (since that includes a decline for players who will drop).
Or let's get more esoteric with Adjusted Net Yards/Attempt probably the best rate stat out there. There he's #3 among completed careers, just ahead of Steve Young, Kurt Warner and Joe Montana.
An 8-time pro-bowler, somehow not including 2008 when he led the league in both passing TDs and passer rating.
He's probably not an inner circle HOFer, but he's definitely in there. Maybe he's more Warren Moon than Joe Montana, but he deserves to be in