CRM

Salesforce. So much the de-facto standard for CRM that its ticker bears the same name. Any company with a sales team (big or small) likely already uses Salesforce. With no other company effectively stepping in to own this category, they will likely continue to subscribe in perpetuity. I am also impressed by their horizontal and vertical expansion via acquisition - Pardot (email marketing), Heroku (Cloud Applications) and Datorama (Analytics) - to name a few.