Leanest android mod? What is leanest android mod out there?
I do not particularly like new Android or Cyanogenmod 12. Too much flashy animations and too much bloatware with tasks running in background.

Or if not specific mod, what are ways of achieving this?
You can disable animations from Developer Options and can uninstall bloatware using System App Uninstaller or Titanium Backup.

The background tasks you mentioned are usually services that can be disabled using Servicely. I've heard that Amplify is also good. 

Note: All of the aforesaid apps require root access.