Superlatives are difficult for me to pin down. When you ask the most, best, worst, top, #1, etc etc, I don't wish to commit to an answer.
Certainly, nothing I find here in the pages of this fine forum makes my top-20 "Greatest Waste of My Time", but with that clearly stated, I can say for sure that it's a waste of:
--time
--energy
--bandwidth
--patience
--screen area
--phone battery when applicable
--cellular data when applicable
--oxygen, as I take deep breaths fighting off the annoyance
...when posters quote entire messages to add a simple one sentence reply. It's worse when the quoted message is long, has many parts, has many different thoughts, takes a long scroll... but it's godawful horrendous when it's pictures, multiple pictures, large pictures, etc etc.
The recent thread where someone asked about experience with leveling a house foundation was extremely interesting and one poster shared his story and added pictures of the job progress and it was a fine addition to the thread. And then at least two other posters quoted his entire post with pictures to add their own reply to the end, just mucking up the entire thread.
Beyond the above listed annoyances of this lazy, unskilled nonsense, it drags down the forum software and servers and forces ever single link in the chain to work harder -- and not only for no benefit, but for the net result of making the discussion more difficult and for more annoying for absolutely every member to read and enjoy.
I've been on a couple forums that give low-level moderator access to a couple active members that log a lot of forum time, especially at odd hours, and gives them the title of Forum Janitor as they work to edit and clean up this kind of lazy/ignorant posting by members who should know better but for whatever reason, don't.
I'm aware that this post may very well have wasted your time if you have read it. But consider that if one of the ignorant has read it, and learns to post with more courtesy, this one post waste of time may actually benefit all of us.
I can dream, anyway.
P.S. Yes, YOU: you haven't invented a brand new joke on the internet when you quote this entire post and add your '+1!' to it. You might well be the turkey that says "a horse walks in to a bar and the bartender says, 'hey buddy, why the long face?'" It may be cute, but you didn't invent it and we've all heard it.