What is the strongest jetstream ever encountered in commercial flight? Jet-streams are high-altitude winds which flow from west to east. They can be provide a very strong tailwind or headwind depending on which direction the aircraft is going.

What is the historic record of the strongest jet-stream ever countered by commercial aircraft? I'd expect the result to be either a very short east-going flight (much shorter than the expected time of travel), or a very long west-going 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