What do you use out of Yeoman/Grunt/Ant for automating your front end workflow and why?

My favorite is Yeoman which builds upon grunt and other tools and provides an opinionated workflow to build front-end web apps.
It is much more than just grunt.
Grunt is a task based build tool which abstracts the boring work into reusable tasks so you don't have to reinvent the wheel in each project.
You can do so much more than just watch for changes.
You can concat, minify JS/CSS, compile CoffeeScript/LESS/Sass, etc.
Recently I have given talk on Grunt, you could find the slides @ http://www.slideshare.net/arif_iq/grunt- And I believe we all know about Apache Ant best suited for Java based projects.
You can also use Grunt with Ant, check some answers at http://stackoverflow.com/questions//integrating-grunt-with-ant

I use ANT.
It is a very powerful tool and have many plug-ins available.
You can automate your test cases/ build process and integration with source safe.