JavaScript!
When one speak JavaScript you may be wondering what JavaScript? because JavaScript has now turn into Pacific Ocean in itself. Picking a good framework in now tough part in web community, because daily new players are entering which are performing better than previous one , somehow. e.g After Facebook introduced Yarn - Node Package Manager, in 3-4 months it's 21k upvotes on GitHub compared to 11k commits of npm ;) One thing is consistent that if you're not up-to-date then in within a couple of months you're outdated! You'll have be all ears to be a good designer/developer. Looking at Google Trends you can conclude how JavaScript has trended over last 12 months.
To really get started you'd learn React, Angular or Vue.js as these are booming very fast in web development, check this survey and this one.
There is a great post of Medium.com to get stared in JS
A Study Plan To Cure JavaScript Fatigue
You'd also read this to know why there are so many JSs out there by Jose Aguinaga .
Stop pondering over what you'd learn, Just get started in JavaScript because using JavaScript you can now create cross platform Web, Desktop (Electron.io) or Mobile Applications(Ionic, React Native, Cordova,etc).
Hope that will help you :)