Lost World:

There's a brief window in time where anime was not readily available on the internet, but was still being produced en masse and at high quality.  This window was around 2000-2009, you could call it the Bermuda Triangle of anime.  For a long time, it was difficult to even discover what series even aired back then, much less acquire a copy of them.  Thankfully, modern technology and sleuthing has finally caught up on this Lost World and presented it to future anime fans:

http://magicasparagus.blogspot.com/p/anime-charts.html

Here we have the name and preview art for every series made since 2000.  Now it's impossible to miss any important series due to how unheralded the series was at the time.  A lot of these series just look bad, via either the name or the art, so there's no need to delve very deep into them.  But a few I haven't seen yet catch my eye:

Tsuyokiss, Tayutama, Rumbling Hearts, Green Green, Tokimeki Memorial, Boys Be, Ojamajo Doremi, Daa Daa Daa, Tales of Eternia, Star Ocean EX, Run-Dim, Sister Princess, Steel Angel Kurumi, Hanaukyo Maid-tai, Figure 17, Strawberry Eggs, Angel Tales, Captain Tsubasa, Kokoro Library, Onegai Teacher, RahXephon, Rizelmine, Tokyo Underground, Happy Lesson, Bandit King Jing, UFO Ultramaiden Valkyrie, Princess Tutu, Petite Princess Yucie, Spiral, Lime-iro Senkitan, Gunparade March, E's Otherwise, Tenshi no Shippo Chu, D.N. Angel, Wandaba Style, Kino's Journey, Mermaid Melody: Pichi Pichi Pitch, Ultra Maniac, Divergence Eve, Bottle Fairies, Yami to Boshi to Hon no Tabibito, Avenger, Mermaid Forest, Planetes, Gungrave, Twin Spica, Peacemaker Kurogane, Gunslinger Girl, Maria-sama ga Miteru, Yumeria, Diamond Daydreams, Daphne in the Brilliant Blue, Hit o Nerae, This Ugly Yet Beautiful World, Koi Kaze, Love Love?, Aisheteruze Baby, Doki Doki School Hours, Wind: A breath of Heart, Tsuki wa Higashi  ni hi wa Nishi ni - Operation Sanctuary, Fafner, DearS, Kakyusei, Kanazuki no Miko, Uta~Kata, Final Approach, MoonPhase, Futakoi, W~Wish, Gakuen Alice, Grenadier, Magical Canan, Ultimate Girls, Bokusatsu Tenshi Dokuro-chan, Ichigo Mashimaro, Izumo: Takeki Tsurugi no Senki, Fushigiboshi no Futago Hime, He is My Master, Koi Koi 7, Honey and Clover, Pani Poni Dash, Kamichu, Oku-sama wa Joushi Kousei, Sugar Sugar Rune, Love Pheremone, Suzuka, Lamune, Black Cat, Jigoku Shoujo, Kagihime Monogotari - Eikyuu Alice Rondo, Magikano, Rakugo Tennyo Oyui, Lemon Angel Project, Kasimasi - Girl Meets Girl, Papillon Rose, High School Girls, Kirarin Revolution, Yume Tsukai, Himawari, Saiunkoku Monogatari, Strawberry Panic, Bokura ga Ita, Mamotte Lollipop, Binbou Shimai Monogatari, Ramen Fighter Miki, Tonagura, Lovedol ~ Lovely Idol ~, Sumomomo Momomo, Sasami - Mahou Shoujo Club, Asatte no Houkou, Gift ~ Eternal Rainbow ~, Otoboku, Saint October, Kyoshiro to Towa no Sora, Gakuen Utopia Manabi Straight, Venus Venus Virus, Rocket Girls, Onegai my Melody, Touka Gettan, Shining Tears x Wind, Sola, Nagasarete Airantou, Kishin Taisen Gigantic Formula, Shinkyouku Soukai Polyphonica, Kono Aozora ni Yokusoku wo, Ookiko Furikabutte, Nanatsuiro Drops, Sky Girls, Moetan, Bamboo Blade, Mokke, Blue Drop, Myself;Yourself, Prism Arc, You're Under Arrest, H20 ~ Footprints in the Sand, Kure-nai, Natsu no Sora, Shikabane Hime, Black God, Maria Holic, WeiB Survive R, Tagami Bachi, 11eyes, WeiB Schwarz, Hatsukoi Gentei, Fight Ippatsu Juuden-chan.

Okay, so more than a few.  The point is any of these could be good, many no doubt are terrible, but none of them received much attention when they came out.  These are not the 'premier' series of the last decade, but the 'lost series.'  Who knows what could be in there?

We know from one example that hidden gems exist -- Happiness! is a perfect example of an unsung hero.  If I investigated all of these series, just imagine how large my 'best anime rankings' could grow!  Well, it's a long term project for sure, but one has to start somewhere.  I think I'll start with 'Rumbling Hearts.'