I dislike implicit 'this' immensely. It is very common for local variables to shadow instance variables, e.g. in a constructor. Habitual use of shadowing can lead to bugs, for instance if you mistype a local variable declaration a later use of the intended name might default to the instance variable. Ruby is my favourite with @foo, it's concise but explicit. Because of this, languages with implicit this tend to have a convention such as "instance variables must start with an underscore", which is just a poor man's sigil.

Here's an example:

In early versions of Swift, using print("hello world") in a UI class would open a print dialog, because it dispatched the print: instance method on NSView, instead of the global print function.