Yep, it's all right there in the title. Thx!
I am not sure how you would find the individual rides with the highest participation, but there are a few organizations that reach thousands of cyclists across the US by holding rides in several communities.
In the US you tend to rarely see huge rides because the logistics and regulation hoops in many areas is to great to overcome. Many rides are capped at 2000 or 3000 riders.
The list below is based on my perception that they hold the larger rides in the most areas:
MS Bike Rides (National Multiple Sclerosis Society): Most are two day rides and many rides attract 3,000 riders or more.
LIVESTRONG (Cancer - Lance Armstrong Foundation): In addition to 3-4 of their own large rides, they have begun to sponsor other large ride like RAGBRAI.
Tour de Cure (American Diabetes Association) Their site reports that between the 80 or so rides they sponsor they get 40,000 participants.
There are a lot of others like Team in Training, American Lung Association Rides, etc, but those three have charity rides as one of the primary fundraising activities and are fairly well known throughout the US.