What's the best way to gauge the success of an anime series? Different factors come into play, from TV ratings to merchandise sales, but one of the most significant ways the anime producers make money is through DVD and Blu-Ray sales. Not every major hit anime is going to sell tens of thousands of DVD volumes. Given the expensive pricing and low episode count of your typical Japanese home video release, extremely popular but extremely long running shows like One Piece aren't going to top disc sales even as they top ratings and merch sales. The shows that do top the sales charts tend to be ones with a set episode count and a dedicated hardcore fanbase. Many of the biggest hits are mecha or moe series, though certainly not every show in those genres is a guaranteed hit. Looking over the top selling anime of the year for each year this century (2015 excluded as those shows haven't all been released to completion, though it's looking like Mr. Osomatsu is going to be the big surprise top-seller), it's fascinating to see which mega-hits are on their way to classic status and which ones have fallen out of popularity since their initial hype.
sales data from http://www.someanithing.com/, based on average sales per volume and ignoring rerelease sales
2000: Love Hina
23,497 average sales per volume
Remember Love Hina? Ken Akamatsu's harem comedy isn't talked about so much today, but in 2000 it was huge.
14,125 average sales per volume
If Love Hina doesn't come up often in discussion, at least people have heard of it! Robo-maid show Mahoromatic is downright obscure today. This is the lowest-selling of the anime on this list.
2002: Gundam SEED
The first Gundam series of the new millennium was only a modest hit in America, but a major mainstream success in Japan.
2003: Fullmetal Alchemist
36,574 average sales per volume
Of all these Japanese top-sellers, FMA might be the most popular stateside. Exact US sales numbers are difficult to come by, but according to the-numbers.com the FMA movie sold 189,557 copies in its first two weeks on sale!
2004: Gundam SEED Destiny
68,959 average sales per volume
Destiny isn't the best regarded Gundam TV series (it scores around .6 points lower than its predecessor on MAL), but it is the best-selling of them.
24,327 average sales per volume
This visual novel adaptation is noteworthy mainly as the start of KyoAni's reign of mega-sellers (while only two make this list, they've had 8 series average over 20,000 sales/volume).
Note: Technically, The World of Golden Eggs at a whopping average of 139,252/volume in that year, but it's hard to say if Golden Eggs can be considered an anime the way Western fans think of anime.
2006: Code Geass
45,423 average sales per volume
Giant robots, battles of wits, Japanese nationalism, and Pizza Hut: the ingredients for a hugely successful series!
2007: Gundam 00
39,353 average sales per volume
The last of four shows on this list (alongside the other two Gundams and FMA) to have aired in the TBS/MBS Saturday at 6 PM primetime anime slot (anime and tokusatsu shows aired in the timeslot from 1993 through the end of 00's first season in 2008).
2008: Macross Frontier
46,297 average sales per volume
The only series on this list that hasn't been released stateside (glares angrily at Harmony Gold).
79,201 average sales per volume
SHAFT's wild supernatural series is the second best selling anime DVD/Blu-Ray release of all time, behind only Neon Genesis Evangelion.
39,503 average sales per volume
71,057 average sales per volume
Still beloved and debated five years after its release, Madoka feels like one of the shows with the greatest chance of long-term popularity.
60,709 average sales per volume
While the Monogatari series sales have slowly declined from 2009's heights, they're still reliable hits.
2013: Attack on Titan
52,052 average sales per volume
After several years of otaku-centric series dominating the charts, the anime industry has another mainstream shonen breakout hit with Attack on Titan.