I think Jim Lee is one these artists that really doesn't need any presentations. If you already read some of the DC, Marvel or Wildstorm comics from the 80's to nowadays, you probably stumbled with some of his covers and pages. In my point of view, he's the best penciller that ever worked for these major companies, a truly master of his tools, always pushing further on style and concept.
You can see some more artworks form Jim Lee at his Comic Art Community Profile or at his DevianArt.
Francesco Francavilla is multidisciplinary italian artist with skills requested by many, he's amazing at almost anything he's working on. Nowadays he's working mainly with Marvel's Black Panther and Captain America and Bucky.
You can see more artworks and updates from Francesco at his Official Website or at his Blog.
You probably know the work of Dave Gibbons by his stunning and majestral artworks on the most famous graphic novel, Watchmen. Although this might be his stepping stone, Dave also did awesome comics for DC and Marvel comics.
Unfortunately, Dave doesn't have a website, but you can see more information about him on his Wikipedia Profile and you can follow Dave's latest projects and news on his Twitter.
Brazil have been one of the top countries on export great talents for the comic book industry. Nowadays we have many brazilians working on major comic companies as Marvel, DC, Image and Vertigo and I'm quite pride of that. As this is a great sign that our internal industry is finally going forward.
André Coelho is one of this comic book artist that had the opportunity to work for major comic book companies, what is quite undeniable when you see the work this inker/penciler has to offer.