I don't know that I can pick any one favorite, it's more like they come and go at different times. My current favorite is probably what I wrote about this week in the general thread, The Moon on a Rainy Night, but that is surely heavily influenced by recency bias.
A handful of other favorites of mine would be (tried hard to limit myself to 5):
I think every single person on the planet should read Yotsuba&! It's a world treasure.
It's not my personal favorite though - that would be The Voynich Hotel. I just love everything about it - great characters, funny as hell, great stories and even a great ending.
Kaguya-sama: Love is War. The manga itself is great but reading it live and discussing it on the old site elevated the experience even more. While I wasn't the biggest fan of the ending, everything else was just so good it's hard for me to give it anything but a 10.
I know a lot of people hate Domestic Girlfriend for the ending, but I thankfully got to bingeread the entire thing during Covid, so I just got to buckle in and enjoy the wild ride.
I also binged Domestic Girlfriend early in the pandemic. I have a love/hate relationship with the series overall. On the one hand, the reason the ending is so incredibly polarizing is because the story and characters are so easy to be invested in. You really care about these three and the melodramatic messes they find themselves in. On the other hand, well, the ending. Domestic Girlfriend makes you feel something, that is for sure; and that is better than a lot of lesser series can say.
I was definitely invested in the characters, I just didn't get invested in whether Hina or Rui won in the end. I just needed to know what happened next. This is why I love the series, because I wasn't one of the people slighted. Plus, I felt the ending hinging on a huge change out of left field was appropriate given some of the previous arcs. I get why some people are made, but I wasn't in their situation.
Three Days of Happiness which is actually the light novel title. The manga adaptation title is "I sold my life for ten thousand yen per year" but I like the LN title more. I recommend reading the author's afterword in the LN if you finished the manga.
Yotsuba&! is great! I wish it got more recognition as a hilarious manga rather than just a stepping stone for Japanese learners.
My favorite though is Kaguya-sama: Love is War. The characters are all insane in the most perfect ways. Skip and Loafer is a close runner-up though; I love its characters for all being so grounded they feel more like people than manga characters. It's a very different feel than I get from other manga.
I also love Yotsubato! :) sometimes it blows my mind that's it's the same author as Azumanga Daioh
My #1 favorite is Nausicaa of the Valley of the Wind, it's several times longer and more complex than the movie.
10/10: Nausicaa, Girls' Last Tour
Almost 10/10: Himegoto: Juukyuusai no Seifuku
9/10: B-gata H-kei, Genjitsu Touhi shitetara Boroboro ni Natta Hanashi (the Nagata Kabi alcoholism memoir), Mata Onaji Yume wo Miteita
Other favorites: Kobayashi's Dragon Maid, Nichijou, Onimai, Melancholia & Nickelodeon, March comes in like a lion, Onanie Master Kurosawa, Gakkougurashi. Maybe parts 6 and 7 of Jojo's