iow: a moment of insane boredom solved. Enjoy. or don't. :) It's not scientific.
Renamon 1974
No surprise that Renamon, the yellow fox from the Digimon series, still holds the number one spot, practically the most popular character for furry art (she is the most popular character overall, though Rainbow Dash made #2 in a very short time and may overtake her. However, most Rainbow Dash art is NOT erotic in nature, and Renamon's lead is still nearly 50% ahead of Dash.) Renamon is alpha slut.
Pikachu 1269
Pikachu, the yellow electric mouse from Pokemon, is very prominent in fandom, yet this is still a bit of a surprise as Krystal is widely acknowledged as the second most abused character in fandom. He/She/It actually is still the third most popular character, just after Rainbow Dash (in most of the erotic art, Pikachu seems to be female). Nevertheless, Pikachu art continues to come out, and most of it is erotic. Pikachu holds onto beta slut position.
Krystal 972
Krystal, the blue fox from Starfox, is a popular furry character, yet she's only the number three erotic character on the site, with less than half the hits of Renamon. She's actually the SEVENTH most popular character, after Pinkie Pie, Twilight Sparkle and Fluttershy (who do not make this list as most of their art is not erotic).
Lucario 833
Lucario, a black and blue fox from Pokemon, is surprisingly high as the fourth most popular erotic character. He seems to be male most of the time, making him the most popular male, as well (although the count includes quite a few female images, too). Lucario tags along right behind Krystal.
Sonic the Hedgehog 821
Sonic the Hedgehog is the very popular Sega mascot, a blue hedgehog with a talent for speed. Sonic is immediately behind Lucario, and is nearly always male, so while Lucario is possibly the most popular male character, should Sonic make up those 12 images he absolutely would be.
Miles 'Tails' Prower 777
Miles is a twin-tailed fox from the Sonic the Hedgehog series, and often depicted with Sonic in said images. He's usually male, but female versions are not uncommon (as furs love their female foxes!). Miles drops a few places again due to ponies, with Applejack and Princess Celestia coming between him and Sonic.
Zangoose 729
Zangoose is a cat Pokemon that I'd never actually heard of before this list, but here he is right after Tails. Like most Pokemon, a bit of a gender bender, but seems to be generally male. His distinctive markings probably earn him a spot in many scenes.
Umbreon 571
Pokemon is a vast breeding ground for slutty furries, and Umbreon also hails from this heritage, not to mention often participates. A black fox-like pokemon with distinctive yellow markings, she's usually female, though tails a fair bit behind Zangoose. She also loses a couple of slots to ponies, with Princess Luna and Rarity between them.
Rouge 560
Rogue the Bat from the Sonic the Hedgehog series was always a bit slutty, even in her original incarnation (being the only Sonic character with breasts must have helped). However, this list betrays her true nature as more of a tease, barely making the list at #9, and worse, being separated from #8 by Derpy Hooves, of all characters. What would you rather see: Rouge's tits or Derpy delivering a muffin? The fans have spoken and the answer is Derpy.
Amy Rose 548
Cute, but strong, and with a series crush on the story lead, Amy comes from the Sonic the Hedgehog world as a little pink hedgehog with a hammer. She rounds out the list of sluttiest commercial furries right behind Rouge. And in some cases on top of her.
So what do we see in the top 10? 6 of 10 have unusual coloration, and 6 of 10, though a different 6, have distinctive fur markings. Rogue is unusual in that she's often depicted with flesh coloring over her body, making it unclear if she's ever furry or just a great set of tits. 4 of 10 are male, which is a reasonably close to even split since all of these characters have been drawn male at one time or another. 4 of 10 characters are Pokemon, and 3 of 10 are Sonic the Hedgehog - both series designed without adult elements at all (well, until Rouge was introduced). Pokemon don't even have sexes, and are referred to as 'it'. Only Renamon and Krystal stand without comrades in this list.
My Little Pony: FriendShip is Magic has made huge rushes in a comparably short time on this list, which is no surprise to anyone who has ever browsed the site or the forums. 9 of them have slipped between the top 10 sluts. Personally I find it refreshing to see some cute, non-adult art now and then, but at the same time it's impressive how quickly they are taking over.
Back on topic, Renamon's lead is commanding, but she carries a lot of non-erotic art as well, someday it would be interesting to skew these numbers by the actual ratio of clean to dirty (though on E621 the erotic art usually outweighs the clean).
Honorable mentions, Fox McCloud, the male fox and hero of Starfox, was right after Amy with 493. Zig Zag, while not commercial, is well known and probably the most drawn, at least most loved of furry's original character sluts, with 335, just one ahead of Felicia from Darkstalkers. We have to go a long ways down to see Lola Bunny of Space Jam (153) or Gadget Hackwrench of Rescue Rangers (144), both of whom I expected to do better.
Updated by Tokaido