The Complete Maus question

Depends on what you're looking for. Did you like Maus? Are you looking for similar, reality-based stories? If so, then something like Stitches might suit you.

If you're looking for excellent examples of the medium, books like The Crow are good. Dark, moody and well-written. From Hell is incredibly detailed and tells the story of Jack the Ripper (though, I must admit, the existential ending threw me).

Then there's the classics. The above-mentioned Sin City series by Frank Miller. Other good ones by the same artist/author are 300, Batman: The Dark Knight Returns #1 (though you don't want to read the sequels...they're awful).

Alan Moore and Dave Gibbons' Watchmen is a classic in the genre (again, forego the Before Watchmen crap).

There's lots out there. I'd suggest finding a decent comic shop in your area and chatting with the employees. There's so much to choose from, and you never know if it will be the story, the art, or the subject matter that grabs you.

I see graphic novels (or comics or whatever) as a form rather than a genre. Compare with novels, say. So going from Maus to Watchmen might be like saying "Oh you liked Wuthering Heights? Then you will perhaps also like Ulysses".

Liking Maus could bring you in a number of directions. Things that might follow on well from it include:

Persepolis - Marjane Satrapi's account of growing up in post-revolutioanry Iran

Fun Home - possibly a leap, but Alison Bechdel's account of her own odd relationship with her father (a deeply flawed human being) might bear comparison with Spiegelman's relationship with his own father.

Pyongyang - Guy Delisle's quirky account of time spent managing a team of animators in North Korea (srsly) is an entertaining read.

Joe Sacco has also written and drawn a ton of books of political reportage and stuff. The Fixer & Safe Area Gorazde both deal with the former Yugoslavia. Palestine and Footnote in Gaza engage with the Middle East.

Sin city?

Maus, or Marjane Satrapi

Maus is far and above better than most other graphic novels. Joe Kubert's Fax from Sarajevo is one of the few that come close.

If you liked Maus, you might try Pride of Baghdad. Y: The Last Man, Vol. 1: Unmanned is good for starting out on. Fables, Vol. 1: Legends in Exile seems to be enjoyed equally by men and women. If you want cartoon fantasy epic, I would go with Bone. If you want to try superhero, don't start with Watchman. Start with anything else. MaybeWolverine and the X-Men, Vol. 1.

You ought to read Hugo Pratt an italian author. Satrapi is good also.

I very much enjoyed Shenzhen: A Travelogue from China and the other books by Guy Delisle. Like Maus, they're nonfiction personal accounts. Persepolis: The Story of a Childhood was also great

Indie stuff :
"Saga" by Brian K. Vaughn and Fionna Staples is an award winning series, and it's just beautiful.
"Sex Criminals" is a hilarious, and very adult, comic, also award winning.
"Mind Mgmt" by Matt Kindt is pretty rad.

Familiar stuff :
There's graphic novel adaptations of classic novels. Marvel did a great "Pride and Prejudice" adaptation. The "Fahrenheit 451" graphic novel is phenomenal.
Also, "Walking Dead" is approachable and fun.

Mainstream stuff :
Marvel, as a whole, is doing some really great things. Check out "Deadpool" for a funny superhero read, everyone loves Deadpool, "Watchmen" is classic, "Batman Year One" has Alfred being a bad*ss, "Batman New 52" by Scott Snyder is good stuff, "Amazing Spider-man" - any and all -Old Spidey New Spidey it's all fantastic and highly recommended, the current "Silver Surfer" by Dan Slott is very fun very Doctor Who-ish, Geoff Johns' "Green Lantern" run is epic, "All Star Western" with Jonah Hex is rad. Also Doctor Who has lots of comics, the more recent ones have better art, and have continuing stories with 10th and 11th. "Guardians of the Galaxy" is a lot of fun too.

Silly stuff :
"Lumberjanes" is totally adorable and fun. "Bee and Puppycat" have their own comic now too, along with "Regular Show" and "Adventure Time". There's a collection called "Marvel Romance" which is a total guilty pleasure and very enjoyable, Stan Lee writing romance - it's top notch.

I suggest just hanging out at comic shops and see what you gravitate towards, what catches your eye, what sounds intriguing. New comic book day is Wednesday, so that's always the best day to go. There's lots of helpful lists on the internet that can tell you what to check out if you're new to comics.