I'm going to take a bit of a different approach and model my picks directly after the presidents on Rushmore...
George Washington: The founder, the first and foremost pioneer, the one everyone looked up to whose shoes could never really be properly filled. To me this is Jack Kirby, but I could see a strong argument for Shuster as well, perhaps a stronger one, I only hesitate because I do think some later Superman artists did a great job as well and thus might have filled Shuster's shoes, Kirby has no true equal.
Thomas Jefferson: Also present very early on, not quite the leader but had a very important roll, everything I found an article on Britannica which suggested Thomas Jefferson was the most famous founding father in his day ie the most well known household name. To me this is tough because there were lots of artists very famous in their day, McFarlane, Neal Adams, Frank Miller, but none of them were super present early on. I think this spot belongs to Will Eisner, he was a rock in the comic industry, many artists looked to him to imitate his style, I think this is very similar to Jefferson.
Abraham Lincoln. Perhaps an unlikely candidate, one that maybe had a divisive or unpopular/not as popular style at the time, but time has treated him very well and he is presently respected and revered. This pick is very tough for me, I want to give it to a silver age artist, I think it could possibly describe Neal Adams, but his work was liked at the time, it's just more liked today, I mean goodness bats 227 has exploded practically just because of the cover art. Gil Kane is also a consideration, Steve Ditko as well, though perhaps Ditko has almost gone the other way. I almost think Matt Baker and Alex Schomburg are in the running, both are heavily collected and beloved today and neither had famous names in their day though they were well employed and their talent was recognized. I choose to give this one to John Romita Sr. I think his work on the most famous character of all time, possibly the second most famous was not as loved in its day, and today I think it's clear he's the one who made the spiderman we recognize.
Teddy Roosevelt: The one who kinda just is famous, he wasn't there from day 1, just very beloved because he had his own style and was always authentic. To me this could describe a number of cult favorites, Mignola comes to mind, Sakai maybe, Ditko again even, Liefeld's name comes up, but I have to give it to Todd McFarlane. His contribution to the comic world is huge and undeniable, he did the cover art for 3 out of the top 10 most printed comics of all time, he created Spawn, worked on Spidey, I think the Toddfather is very deserving of the list.
TLDR: Kirby, Eisner, John Romita Sr., McFarlane