Or could be rephrased to something like "what message impacted you the most?"
For me it's these 2 with no competition
One Piece
The Horizon
Blue Period (for personal reasons though)
One Piece
Jumyou wo Kaitotte Moratta Ichinen ni Tsuki Ichimanen de
Naruto (I was a weekly reader when it ended)
Sakamichi no Apollon
these are the ones that have had the most emotional or personal impact on me
Our Dreams at Dusk Shimanami Tasogare
Ano Yoru no Pool
Blue Period
don't think i'll ever recover from these
Shimanami Tasogare
Blue Period
Ao no Flag
Boy meets Maria
Niku wo Hagu
Without a doubt
Berserk
I would definitely not be the same person I am today if I didn't read it. R.I.P. Miura.
Right after that are :
Oyasumi Punpun
Helped me get my shit together s a teenager.
Vagabond
Helped me finally calm the hell down.