"Best" depends on the scenario. There are times when you only care about the smallest possible single bundle, but in large apps you may have to consider lazy loading. At some point it becomes impractical to serve the entire app as a single bundle.
In the latter case Webpack is generally the best way since it supports code splitting.
For a single bundle I would consider Rollup, or the Closure compiler if you are feeling brave :-)
I have created samples of all Angular bundlers I've ever used here: http://www.syntaxsuccess.com/viewarticle/angular-production-builds
The code can be found here: https://github.com/thelgevold/angular-2-samples
Angular version: 4.1.x