The Flash is not an easy character to draw. I say this not from experience (as pretty much every comic character ever is not an easy character to draw if you're me), but from as empirical-as-I-can-get data gathered from years of reading interviews like the ones in Keith Dallas' wonderful Flash Companion from TwoMorrows (to which I was an enthusiastic contributor). While the costume and general aesthetics may not be incredibly difficult, most artists I've seen or heard accounts from state that super speed is just flat out tough to render visually on a static page.
However, despite this handicap out the gate, the Flash character in its various forms from Jay Garrick to John Fox and all points in between has been the fortunate recipient of many incredible draftsmen who have created truly dynamic and imaginative art featuring the Scarlet Speedsters.
Here are my biased favorites.
Honorable Mention: George Perez
I feel like it would be a cheat to sneak my favorite artist of all-time on here by virtue of his drawing the Flash as well as he draws every other character in comics, but he did contribute greatly to giving Wally West a unique physical personality in New Teen Titans and gave Barry Allen perhaps the best sendoff in comics history during Crisis On Infinite Earths.
Honorable Mention: Brian Bolland
To the best of my knowledge has never drawn the Flash on any interior pages, but sweet sassy molassy were his covers gorgeous.
5. Karl Kerschl
I'm still waiting for an extended stay on a Flash title for Mr. Kerschl, but for now, I'll take what I can get with his beautiful work on Wednesday Comics. Karl's linework is so soft and smooth but with just enough edge to know he means business, and it works perfectly with this character. His Flash, be it Barry Allen or Wally West, is exploding with emotions, good and bad, and seems to really enjoy his work, but also take his job quite seriously--perhaps a commentary on the artist himself? Regardless, when I read a Karl Kerschl-drawn Flash story, I feel like I'm racing alongside the characters, and there's not a lot more you can ask for.
4. Alan Davis
You may hear the name "Alan Davis" and not immediately think of the Flash as one of his signature characters, and you'd have a pretty strong case, but remember that he was the cover artist of Wally West's ongoing series back in the 90's--granted only for eight months, but it left an impression on me as I was just seeing the book on the stands. Really it was Davis' labor of love project Justice League of America: The Nail and its sequel, where he shined as a Flash artist, bringing Barry Allen straight out of the Silver Age and into the 21st century. Alan's art is flat-out gorgeous no matter what he's drawing, but his style has a certain fluidity that's really perfect for a character like the Flash; in his afterword for the collected Nail, Davis talks a bit about how he really put thought into making Barry Allen look like a runner, and not just a generic muscleman, and you can see that every time he draws him.
3. Mike Wieringo
If I have any major regrets as a Flash fan, it's that I didn't get to truly enjoy Mike Wieringo's time with the character because I wasn't reading the book at the time and have never been able to track as many issues down as I would like. I became a fan of Ringo through his later work, most notable his time on Fantastic Four, but I've seen enough Flash commissions and covers by him over the years to fall in love with his rendition of the character and consider him one of my favorites by virtue of his work on a title I didn't even read. By all accounts, Flash is where Mike really grew and came into his own as an artist, and it's fitting given how much the adventurous and go-lucky persona of Wally West matches up with everything I've ever heard about him. His Flash had both an energy and sense of fun that I think has stuck with the character and influenced on some level just about every artist who has come since.
2. Carmine Infantino
Without Carmine Infantino, there would be no Flash as we know him, simple as that. He designed the costume, he designed the Rogues, and he innovated the best methods as far as portraying the character's powers and feats. Infantino first made his mark on the Golden Age Flash, Jay Garrick, but really came to prominence working on Barry Allen, avatar of DC's Silver Age, as well as his friends and enemies. It was Infantino who if he wasn't the first to do it really perfected the idea that you could show a speedster using his powers not just by creating a trail of smoke behind him or drawing blurry lines all over him, but actually depicting the character at various stages during his run and connecting them to create the illusion of a man who could be in several places at once. It was this work ethic that really distinguishes Carmine Infantino as a guy who not only refused to take shortcuts, but who achieved impressive tenure by drawing the Flash's adventures for over a dozen years in two stints that were separated by nearly a decade and a half.
1. Scott Kolins
I don't think it's a stretch at all to call Scott Kolins the modern day heir to Carmine Infantino's throne in a 100% complimentary way. During his two-plus year stint on the main Flash title and in his speedster-related follow-up projects since, Kolins has thrown himself into the detailed portrayal of super-fast motion with a vigor I don't believe any save Infantino ever have. Kolins doesn't simply draw speed lines or a few blurred images, he takes the old Infantino trick of placing a fully-realized figure at each point of the Flash or Zoom's arc of motion and then filling in the spaces between with effects. The reasons Kolins tops my list are partly sentimental--he illustrated the period which saw me truly embrace the Flash as my favorite DC character--but also practical, as he took what came before him and refined it with modern techniques and technology. Kolins' characters, and in particular his Flash, have a buoyancy you just don't see anywhere else, a vibrancy and energy that is unmatched, and with the Fastest Man Alive, that's the guy you want providing the pencils.