I wouldn't use HHVM as it is now not supported.
Use PHP7 if your plugins and themes support it.
I use FastCGI Cache e.g. --wpfc
Then I also manually setup REDIS object cache. To do this just create a fake site with --wpredis to activate it. Then delete it. Then add the redis plugin and activate and you have both working. This gave me the best performance. Make sure you disable object cache under w3tc though.
