Helm is a little heavy-weight. Ivy is lighter, but also does a lot of NIH stuff it doesn't need to. I personally like selectrum, (https://github.com/raxod502/selectrum) because it provides the features of Ivy and quite a bit of extensibility, while still only using the built-in extension points in emacs (completing-read-function, etc.). I do wish the emacs developers would bless something and include and configure it by default, though.