While Peyton might have shown more signs as to becoming a prolific passer his rookie season, his INTs really hurt the team, something QBs like Flacco, Ryan, and Roethlisberger avoided.
To say he had the best rookie season for a QB and that no one did any better is asinine. You can have the fastest and most impressive split times in a race but if you don't cross the finish line then its all for naught. Flacco and Roethlisberger made it to the AFC Championship game in their respective rookie seasons. Sure, they didn't put up super impressive numbers to get the team there, but they did make the throws asked of them and avoided the ones that would have hurt their team.