What is the lightest custom Android ROM (Android 4.0 or higher)?

I need a minimalistic custom Android ROM which has a lot of RAM free for use because the app that I need to use needs a lot of RAM. I want to only use my phone for one thing and only have one application installed on it. I will install the app after I install Android on my phone. The app is using GPS and needs Android 4.0 or higher. The name of the app is Map My Tracks. What is a custom Android ROM that is the lightest of all? My phone is HTC Wildfire S.