What is the strongest jetstream ever encountered in commercial flight?
Jetstreams generally don't stay perfectly straight. They tend to follow a meandering, curved path so it would be rare for a flight to stay in the max tailwind for very long. 

Dispatchers will often take advantage of jetstreams and plan the routing to follow part of the jetstream core. 

There is a website called Ground Speed Records where pilots can post photos of high groundspeeds for an "Unofficial" record. 

Here is an example from that website giving a groundspeed of 736 knots from a 240 knot jetstream tailwind. That is probably about the highest jetstream speed you will see:  B747-400 Ground Speed Records