I’m a huge fan of Heroku (PaaS), AWS (IaaS), as well as Intercom (SaaS). Heroku has helped me scale infrastructure and hosting easily, while Intercom has been a huge help with user retention and onboarding (my main usage is for new/existing users, not leads). Other SaaS products I use that I enjoy are Raygun for error reporting, and Papertrail for storing/accessing logs.