In 2015, I have flown:

International: AC J, UA F, LH F
Domestic: AC J, AC Express Y, UA J (and p.s.), DL F, LX J, LH J, SAS Plus

In terms of rankings, I'd say for international product:
LH F > UA F > AC J
for service:
AC J > UA F > LH F
LH had really surly and rude FAs who hid in the galley or chatted with passengers in the other aisle, leaving me to fend for myself. And they told me my expectations were too high, and they were sorry if they couldn't compete with other airlines, like Air Canada, in ensuring that I wasn't sitting around for 30 minutes without having a crew member come by or being able to go to the galley to find one. They also told me to leave the galley once when I wanted a drink, and had been waiting at my seat for a while.
UA FAs left the F cabin as soon as they decided everyone should be asleep. One was found knitting in the mid-J cabin, which is where I had to go when I wanted anything for the rest of the flight.

Domestic, I prefer AC for their generally friendly FAs. Other than that, it's all the same really, except SAS Plus (but that's not a J product). I like UA p.s. ice cream sundaes. I like lie flat seats. I dislike blocked middle euro-J, but I can live with it.