Seriously. I somehow ended up using all of them, and I honestly don't know what's going on anymore. I thought my Talk got merged into Hangouts, if I start a chat in Gmail it's sometimes findable in my iOS hangouts app, and sometimes it's not. A while ago the Voice team asked me to merge Voice with Hangouts, which I did because clearly the service was dead. This seemed like a plan... your SMS chats are just chats in Hangouts and in addition to calling other Hangouts users you can call phone numbers, but we're keeping the Voice app & site around for legacy users. Fine. But in January they released an upgrade to the Voice apps with this gem:

If you currently use Hangouts for your Google Voice communication, there’s no need to change to the new apps, but you might want to try them out as we continue to bring new improvements.

W.T.F? Don't bring me into your internal office politics...

Then there's Allo because with all the above they made it too complicated to chat with someone or something. Also Duo, because video calling was completely missing from the offerings above (it wasn't).