I'm 21 this May and I've been reading comics all my life but I didn't start following story lines until the Death of Superman, then in 1996 Kingdom Come was given to us, it is my first Limited Series and is still my favorite "independent" story. I make a point to read it every year, one chapter at a time from May to August like I did when it first came out. Newer stuff though I would have to say Whedon/Cassaday's Astonishing X-Men, House of M, and Blackest Night. (side note: the Booster Gold Blackest Night tie ins have been mindblowing and if you haven't read them you need to.)

a random list of mine would include

Age of Apocalypse (original)
Days of Future Past
Infinity Gauntlet
Kingdom Come
Green Lantern:Rebirth
Justice
Universe X
Fables(entire run)
The Flash (Geoff Johns run)
New X-Men (Grant Morrisons run)
The Authority (Morrisons and Ellis' runs)
Top 10
Watchmen
Mouse Guard
Thor (Walt Simonson run)
Ragnarok

i'm sure theres more but i can't think of them right now

DC NEW FRONTIER!!

or

THE SINSTRO CORPS WAR

are my absolute favorites!!!!

Crisis on Infinite Earths of course!!

Identity Crisis. That story was perfect for a comic book. The art was great, and the writing was flawless. It was steeped in continuity, but still really accessible to someone with a limited knowledge of the DCU. I pick it up again every few months and I get something new out of it every time.

Funny enough, I just picked up Day of Vengeance again the other night and was thinking the same thing. I'll presume you've read it, but if not it does include the next step for Jean Loring.

These are tough choices to make, but I'd have to say that the following are my top 10, in no particular order.

1) Watchmen
2) Batman: The Dark Knight Returns
3) Kingdom Come
4) Marvels
5) Swamp Thing: Dark Genesis (Len Wein & Berni Wrightson)
6) Incredible Hulk (Peter David run)
7) Mighty Thor (Walt Simonson run)
8) Saga of the Swamp Thing (Alan Moore run)
9) Astro City (Kurt Busiek, mainly 1st mini series)
10) The Sandman (Neil Gaiman, entire body of work)

I think the Kingdom Come end is Brilliant.