"The four p5 wins this year are nice but beating ranked teams is also impressive."

Now you're doubling back and realizing that 2 of those teams were ranked at the time. The fact is nobody really knows how strong a team is in early season, so we'll have an idea about this schedule's strength at the end of the season.

As far as the 2006 and 2009 teams go, here's their record against teams with 8+ wins (will include the losses and bowl games if below the threshold):

2006
@Arizona (6-6) 13-16 L
vs. Tulsa (8-5) 49-24 W
@BC (10-3) 23-30 L
@TCU (11-2) 31-17 W
@Utah (8-5) 33-31 W
(bowl) Oregon (7-6) 38-8 W

Summary: Stumbled out of the gate in game #1 on the road. The most challenging part of the schedule was on the road. TCU was the best road win of the Bronco era. Both losses by one score in the first 3 games.

2009
(neutral) Oklahoma (8-5) 14-13 W
vs. Florida State (7-6) 28-54 L
vs TCU (12-1) 7-38 L
vs Air Force (8-5) 38-21 W
vs Utah (10-3) 26-23 W
(bowl) Oregon State (8-5) 44-20 W

Summary: Came out hot on a neutral field to get great exposure and jump up the rankings. After the catapult to the top, suffered 2 home losses of 4+ TDs. Not a single difficult away game the whole season. Very impressive close out to the season.

I just can't get behind a team being Bronco's best that lost by 4+ TDs at home twice. The road schedule in 2009 was an absolute cake walk. There really isn't a measure that the 2009 team was better than 2006.