I don't typically use the forums here but I'm gonna try to become more active (fingers crossed!)
My most used app would technically be runtastic Pro because I run for about an hour every day, but actually physically interacting with an app it would have to be WPCentral or Weave
everyone here uses whatsapp... cant run away from this
My skype is always on, and off course, thirdly, .... it has to be Store... we need this to get all the updates and stuff!! LOL