I'm a beginner when it comes to customizing my home/lock screens but I absolutely love Nova, so much so that I went Prime. I like a clean, streamlined look to my home screen and Nova lets me do that without being heavy on resources.
My vote goes to SwiftKey. It's very intuitive and after about a week of usage, it had learned not only my particular brand of slang but it also learned the way I mistype things. SwiftKey Flow is also fun. Added bonuses to SwiftKey is that it supports multiple languages and the themes are nice.