Best synonyms & antonyms dictionaries according to redditors

Yeah, I believe it was this one: https://www.amazon.com/Merriam-Websters-Dictionary-Synonyms-Discriminated/dp/0877793417

Bill Bryson's Dictionary of Troublesome Words. It's a slim volume, but it covers all the but the most obscure usage cases. Especially useful if you need to know things like the difference between enormity and enormousness (personal pet peeve).

There's a chapter in Strunk and White on commonly misused words and phrases, if you just need to brush up on the basics.

I have this book at home, which does exactly what you describe. I found I haven't used it that much, but it has been helpful when I've needed it:

http://www.amazon.com/Merriam-Webster-Dictionary-Synonyms/dp/0877793417

This is something that I remember being similar to what you described, though I couldn't locate the exact edition you mentioned:

DK Visual Dictionaries

If you look in the "customers who bought this product also bought" section further down there are other examples. DK published a lot of these the most familiar to me being a Star Wars one published when Lucas started making the new trilogy. I hope this is it, if not good luck!

I used that word (well almost) in my 2nd ever reddit post in /r/fifthworldproblems . Back then entries with obscure words were all the craze.

I leaned very heavily on my copy of Dictionary of Uncommon Words by L.Urdang for that one!