Rollup [1] seems to be pretty good at tree-shaking and apparently babel's babili minifier [2] is better than UglifyJS (but I guess it's too early to tell)

But both come with their own tradeoffs: Rollup is ES6 only and doesn't support certain features from TC39's Stage 3. Babili is very very new :)

[1] https://rollupjs.org

[2] https://github.com/babel/minify