YCM is great. Depending on the language though you may need additional plugins to get "smart" completion into Vim. For instance, tern.js does this for Javascript:

[1]: http://ternjs.net/

YouComplete does not currently work with neovim as neovim does not ship with python bindings yet.

It's worth noting they appear to be working closely [1] with Valloric to ensure that vim plugins will easily work with NeoVim.

[1] https://github.com/neovim/neovim/pull/582

Sure, but unless I'm misunderstanding the parent commenter they were asking for a vim plugin, not a neovim plugin right?