I  have a multiplayer game and I don't really know where to host it and  which Online service to use if you have any tips, advice or new services  to propose.  
The game is a RTS like, 1v1 or 2v2.

**Key features:**

* Matchmaking
* Party
* Friends invite(via online system)
* Leaderboard

I will maybe make all the leaderboard and user information part myself with lambda and a database.

**Hosting service:**

* Gamelift(AWS)
* Agones + Kubernetes(GCP), I love this project and work a lot with K8s but maybe overkill and time consuming.

**Online service for matchmaking:**

* Epic Online Service
* Steam Service

The Engine I'm using is Unreal Engine 4
I'm also trying to figure out exactly the same question

Besides the options OP mentioned, I'm also looking into: 

* Playfab(has most of the multiplayer / social features)
* Photon (seems to have better integration with Unity, but does have Unreal support as well)
* Open source projects (e.g nakama) or building my own backend