Does it count if the thread was started before this year but was very active in 2018?I haven't contributed anything because I don't feel qualified to add to the high-level discussion going on there, but I've really learned a lot from lurking on the Top Is In thread: https://forum.mrmoneymustache.com/investor-alley/top-is-in/
I had a baby this year and have been trying to simplify my house. I love the "Getting Rid of Stuff" forum this year and have found it very motivating. https://forum.mrmoneymustache.com/throw-down-the-gauntlet/continued-getting-rid-of-stuff-clearing-out-clutter!-part-2/
I'll start. My nomination for the best is @APowers' thread on keeping his grocery budget under $200 a month for a family of four. It's a master class on grocery shopping -- with facts, figures, and photos. No matter how you shop, how many you feed, and what you eat -- you can learn something from this. https://forum.mrmoneymustache.com/share-your-badassity/have-a-sub-$200month-grocery-budget/What other great ones are out there?
OK it's now 2019, but I want to put a word in for the humble but powerful thread started in November 2013 by MrsPete: https://forum.mrmoneymustache.com/ask-a-mustachian/what-small-things-did-you-do-today-to-save-money/Great stuff.