The first thing to remember is that everyone's sensitivities are different. Some of us react poorly to chemical sunscreens, others have allergies to essential oils, preservatives can break people out, etc. Which means that there's no one "this definitely won't irritate you" product out in the universe. However, there are several formulations known to be gentle, and if you try a few of them, there's a high likelihood you'll find one that works.
It's only recently become available in the US, but we've been hearing rave reviews about Bourjois Healthy Mix Serum Gel Foundation for years now. It's light, non-comedogenic, and the finish is beautiful. If you prefer compact foundation to bottled, Vincent Longo Water Canvas is a creme-to-powder formula that is legendarily friendly to acne-prone skin and anyone else who needs foundation that "breathes." Some skin types also react poorly to brushes or sponges, which is one of the reasons (besides its mild formula) that the Diorskin Airflash foundation spray gets rave reviews.
Have you discovered a foundation that's perfect for sensitive skin? Let us know in the comments—we'd love to hear about what works for you and why it's so great!