Doctors Without Borders just because of their ability to get care to almost any part of the world. I consider my contributions to it a form of compensation for my tax dollars that go without my consent to fund our hungry war machine.
With the recent US tax law changes, I created a charitable giving account via my brokerage to bunch up donation contributions to the account and then parcel out this account balance during the intervening months. Edit: this also allows anonymous contributions, which tends to defeat some charities' tendency to send unsolicited commercial mail.