I voted for the web because its historically true (and by the numbers even true today… far more web apps than mobile and desktop combined). However, these are all client technologies and, you maybe noticed already, these things tend to simultaneously stay the same (render boxes on a screen) and change all the time (Flash, WPF, JavaFX, JS-framework-du-jour).
Backend dev is going through a seriously interesting renaissance right now. The Cloud™ has become the standard delivery vehicle in the past decade. This wasn't super exciting because things were not fundamentally different. Just someone else owned the machine machine.
We still deployed on the metaphor of servers by way of virtual machines and, more recently, containers. And now very recently vendors are shaking up the entire premise of backend with a new metaphor: functions as a service. These include AWS Lambda and Azure Cloud Functions. (Others exist but MSFT and Amazon are winning adoption by a significant margin.)
I spent the past decade of my career somewhere between mobile and the web. I intend to spend the next decade on functions as a service. Food for thought!