gmail for the power of it's breakthrough tagging (label) interface, brilliant filters/actions, conversation format, best-in-class spam filtering, and lack of restrictions like blocking POP3. also b/c it includes google docs, stand-alone maps, notebook, g talk (the stand-alone BB app is great), and web site analytics.
yahoo for mom to use to send cute e-cards with dancing cats to me - which inevitably results in junk mail. i find yahoo to be cumbersome and the ads ubiquitous and maddening.