Personally it would be claymore, often hailed as a female centric berserk.
A story about the lowest class of a generation of engineered superhuman woman fighting monsters to save humanity.
The art gets better and better as it goes on, with stunning panels everywhere(example below).
The intrigue struggle and strive the characters go through is a stellar example of simple yet effective storytelling.
Claymore is great.
I think I’d pick Sweat & Soap. It’s a really good romance manga with great characters. I think it’s the quintessential romance manga with adults (Wotakoi being a close 2nd and actually my preference, but wouldn’t consider it as quintessential). It doesn’t fall into a lot of the traps like many other romance manga tend to and particularly the journey of the female lead is really good. The initial premise seems like it’s pretty fetishized, but it’s not at all.