2D games; Javascript or C#?

Javascript, like any scripting language, is easy and convenient for fairly small projects.

For projects of any serious size or complexity however, you need the organizational power of a real programming language such as c#.

Just about every full Unity game project I know of that started out in javascript, eventually had to move to C# to complete.

(Btw, its technically not Javascript, its Unityscript. Javascript and Unityscript are two different distinct dialects of ECMAscript. Unityscript files by convention end in .js which confuses things somewhat.)