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They've rolled out a new version of video/voice calls this month[1] which supposedly improved the quality quite a bit. I haven't personally tried it yet, though.

[1]: https://whispersystems.org/blog/signal-video-calls/

I was recently surprised by a Signal call and the quality was surprisingly good compared to my previous experiences. I would give it another try.

As does Wire, just for the record.

They are one of the newer kids on the block. Open sourced in March 2016, End to End encrypted (everything and by default) since June 2016 (or the other way around). Supports chats, calls, video calls, file transfers and multiple devices (including Linux, though Electron...).

And when I tried it, for me seemed to have the best call quality, it was better than my network call quality.

Server isn't open sourced yet... I saw a video where they claimed "Q1 2017", but that doesn't leave them much time...

Call quality is much better now. I believe it's a combination of direct peer to peer stream support and a switch from Speex to Opus.

Newest version has improved quality immensely.

Their latest build has WebRTC calls, which improved quality dramatically.