Let's not forget Teteboro, NJ and the New York Approach area. 4 of the World's busiest airports are within 20 miles of each other. JFK, LGA, EWR, and TEB.
You are correct that SOCAL is the busiest TRACON, however they are also the largest (area wise) TRACON operating from San Diego to Santa Barbara.
I believe you are correct in stating that VNY is the busiest GA airport in the world. SMO is also right up there.
I believe that (I'm on the road, no info) Scottsdale, AZ is the 2nd busiest single runway airport in the US (SMO is 1). It gets most of the corporate traffic going into Phoenix and a lot of Flight Schools are based there.
Shaun3000, I think the airport you are referring to is Denver Centennial (APA), it's got 3 runways, but is a very busy airspace since it gets most of the corporate traffic going to Denver and also has numerous flight schools.
How about Houston Center during the Summer T-Storm season, I seem to be constantly amazed at how well they handle all the reroutes and deviations.
As always, ATC normally does a great job handling these all busy airspaces, flying in and out of them is a very easy if you do a little homework and plan ahead. Remember to look at the whole airspace and not just your planned route. It makes it easier for everybody when you filed for the LEESE arrival, then 100 miles out they give you "Direct Jesup and the remainder of the BITHO7) that you know where Jesup is.