Well, I would like to mention some of my favourite comics too.
Some of them have been mentioned before but most of them not.
For starting I have to talk about some oldies. Comics like Herriman's Krazy Kat and McCay's Little Nemo following with Segar's Popeye and Schulz's Peanuts.
They have a mixture between humor and poetry that sometimes I miss it.
In this fashion I would like to mention the most recent Calvin & Hobbes by Bill Waterson and The Far side by Gary Larson. Chris Ware needs a category of its own - maybe blue and poetic - and among the funnier ones Mark O'hare's Citizen Dog.
I've never been a Heroes fan but certainly Will Eisner with his Spirit and Mike Mignola's Hellboy deserve to be in any list. The former showing what it can be done with a hero in seven pages - and that heroes can be funny too - and the latter demonstrating that there's another way for illustrating heroes, with plane colors, bolder lines and less details giving a really dark and less fancy atmosphere to them, revitalizing the horror gender.
From Europe, first I have to talk about Spain. Ibánez and his Mortadelo y Filemón comic, and the new guys like Manel Fontdevilla, Gallardo, Max and the not-so-news Carlos giménez, Daniel Torres - and his postmodern hero Roco Vargas - and Pellejero/Zentner's Dieter Lumpen adventures.
In France I have to mention Jacques Tardi's comics - mostly Adčle Blanc-Sec series. In Belgium, besides Hergé, I like the works of Ives Chaland - with his Freddy Lombard - and Franquin.
He's not really a comics ilustrator but I really love Ever Meulen illustrations - with his Escher style - and if anyone knows about a book with his work I would like to know about it. By the way he's dutch.
From the Underground verge I like The Fritz Cat by Robert Crumb and being on a Lucasarts forums I must recall Sam & Max by Steve Purcell.
Among Manga comics I would like to mention Dr. Slump by Akira Toriyama.
Finally being born over the River Plate I must to talk about people that have been working on argentinian magazines and papers on the last decades.
Argentinians like Quino and his Mafalda, Fontanarrosa and his gaucho Inodoro Pereyra, Carlos Trillo and Cacho Mandrafina, the uruguayans Tabaré and Alberto Breccia - with his argentinian son Enrique -, the italian Hugo Pratt - with Corto Maltese - and mostly all the people that used to work on Humor magazine - a kind of Mad magazine - against the militar dictatorship.
That's pretty much a very accurate list of my favourites.