The mix up is usually because these extensions are about two unrelated technologies: opcode caching and key-value data store. For WordPress you prefereably want both. Opcode caching is really the “normal” way to run PHP (and lack of it is essentially crippled shared hosting way). Data store can *(and should) be used by WordPress object cache plugins for better persistent caching.
So out of those you named:
- APC is opcode cache and data store
- APCu is only data store
- OPcache is only opcode cache
Since APC is older, at the moment you likely want OPcache as well as some data store, not necessarily APCu (although it is perfectly fine choice).