Crazy pitch, but the complete Hate run (even if it's technically ongoing). It's one of the most in depth, generation spanning character studies I can think of.

Also, Watchmen is so fucking good. In 12 single issues you get:

Complete alternate reality world building
Thorough character lore
A genuinely good mystery
A complete, coherent story with a beginning, middle and and (you can't take that for granted at this point)
A masterclass in the potential that comics uniquely hold for making text, art and page layouts intertwine to enhance the story

I try to read it once a year. Every time I do, something new pops out at me. I feel like there's a shell game going on where if you pay too much attention to one single element of the comic, you miss out on three more happening simultaneously. I have no problem calling it the best comic ever written, in the sense that the Rolling Stones are the best rock band, Godfather is the best movie, etc. Like, it's a boring answer, but it's not entirely wrong.