a) background noise reduction
b) echo/feedback cancellation
c) input latency
I assume input latency may not be such a big issue anymore, with ultra low latency codecs (presumably) being widespread now.
Feedback cancellation is a problem if any side of the conversation is using speakers, rather than headphones. I guess chat services meet at the lowest common denominator, making fairly conservative assumptions and often cutting audio aggressively.My personal list, best to worst, in respect to my requirements above:
Google Hangouts
Jitsi
WhatsApp audio call
Skype