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.
There is no definite answer for this but here is my point of view - 
The Simple Solution - SlimRom:
SlimRoms - On-Click away,free, HTC compatible ROM.
With SlimRom you can configure how much of google services you wish to use which have significant effect on device performance. 
The Hard Solution - Create One:
You can create your custom Android OS and build clean version of it (without any google app like chrome or youtube).
If you really want to get wild you can delete all unnecessary apps under platform/packages/apps (e.g calculator,camera).
If you choose this option bear in mind that HTC specific features won't work.

More data:
If you're still not sure and need more data about specific features that you'll need in your app check the Custom ROM Comparison Database.