My GUESS is that the 85mm 1.2 L is the lens least likely to get an update within five years.Not so sure. The 85L has two weak spots that Canon should act on; its one of the non-IS "L" primes that encounters most failures (electrical problems) and the sloooow focus (even if its better than the 85LI). Its esspecially sad becasue it limits the use of the lens for action shots at night.
There is also the reflection issue. Those who use the 85L at night will surely have seen occaisional "sparks" of green light from time to time. I expect this can be fixed with some of the new coating techniques Canon continues to develop.
I would expect a 85L III to see improvements on all three issues.
When Canon starts to release new bodies with better AF and software for focussing such as eye recognition the usefulness of the 85L will improve as we can begin to shoot more confidently @F/1.2.
I used my 85II for a lot of low light weddings here in the UK using nothing more than 5DII's centre spot. I used a fine focus screen to assist me and it worked really well. I generally got about 3-4 shots per wedding which wasn't as sharp as I wanted due to focus errors, that's out of about 2000 image taken. On the 5DIII, my average hasn't changed but the single point spot focus is needed to get the best out of this lens but keepers vs losers is the same.
Many people complain about this lens, if you can't use it properly...go back to f2.8 glass and be happy. if not, then stick with it, it's a tough tool to learn but it's worth it. Top tier glass like this which has a thin DOF is always going to require a very high degree of skill and accuracy. Don't blame the lens...