Deadpool. I like each series for different reasons and there are things I don't like in each series as well. I've liked the character since I learned of him in the 90's and think he's still a great character
X-Men, my favorite since I was a kid.
Preacher, hell of a lot of fun and an interesting concept.
Transmetropolitan: I love the mean spirited misanthropic social commentary shown through the veil of the future/sci-fi.
Ms. America, because I like the fact that there's a comic I can read if I ever need that one last push to kill myself.
Daredevil. Miller, Nocenti, Brubaker, Bendis before he sucked. Waid was pretty good on it too.
Promethea. Best latter-day Moore work IMO.
The Invisibles & Animal Man. Morrison has (many) faults as a writer, but I love his stuff when he's on. @Detrogen mentioned The Maxx, but I'll mention it too. Probably did more to change my attitudes about comic art than any other series.
Kinda obvious ones, but worth mentioning: Bone, Sandman, Watchmen, V for Vendetta, Ronin. The usual suspects.
Ranx - This is one I discovered through Heavy Metal magazine. I love the art, I really like traditional media being used in comics.
Anibal Cinq: The Last Ten Women I've Known - Also something I found through HM, it was written by Alejandro Jodorowsky so it's fucking weird. Ditto the cool art.
FAKK 2- as adapted into a comic by Simon Bisley. I haven't seen the movie in a long time but recall it was wasn't that great, but the comic is fucking badass. There might have been more than one comic adaptation, but the one I read appeared as a special edition of Heavy Metal magazine back in '99. Related to this is Melting Pot which was a Bisley-Eastman collaboration that lead to FAKK 2/Heavy Metal 2000 being created.
Tintin because who doesn't like Tintin?
Maus- The story is great, art is great, I would really like to check out more by Art Spiegelman since he's a pretty big underground comix guy. I like funky dudes like Crumb.
Watchmen- 'Nuff said.
Arkham Asylum- This is cool becasue you've got a very well-known mainstream character like Batman, but a very unconventional (for comics) art style.
That one Batman comic where the Joker says boner a lot
Squee's Wonderful Big Giant Book of Unspeakable Horrors- I don't care if it's edgy-13-year-old fare, I still love it. Partly becasue it was the first thing to push me away from manga in terms of both interests and art.
While I don't consider myself an expert on Batman, I think that the best Batman stories are the unofficial trilogy formed by Year One, the Long Halloween, and Dark Victory with TLH being the best of them. The Killing Joke and The Dark Knight Returns are also pretty good.
I really like Watchmen, but I don't really think that V for Vendetta is as good.
In terms of long running series, the ones I've enjoyed the most so far are Fables, Y: The Last Man, the Boys, and the Walking Dead (though I haven't finished the Walking Dead yet).
I also like the Cape by Joe Hill (the original story, not the extra stuff they threw in).
If I had to say what series I enjoyed the most the previous year then it would have to be the 2019 Red Sonja series. I thought it was so good that I started reading the older stuff.