Looking for Anime Games to play

Hyperdimension Neptunia Series?

I seen that. One of my friends play it and enjoys it, but idk

YS series, especially if you go for 100% completion on the bonus modes

why don't you try Valkyria Chronicles?

If not on steam only, then Ar Tonelico 1 and 2 (PS2 game, you can play them on PC with pcsx2), Atelier series (1,2,3 are on PS2), Mana Khemia (1+2 on PS2)

http://store.steampowered.com/app/251150/ its extremely good jrpg

Valkyria Chronicles
It's a tactical strategy game, part turn-based, part real time, all anime

https://en.wikipedia.org/wiki/Arc_the_Lad Kinda oldie, but was good :)

I highly recomment Recettear.

Sakura

Rune factory?

Very long and in-depth JRPG based on a popular anime from the 2000s:

View attached image.

http://store.steampowered.com/app/330390/

Grandia 2

It's an old JRPG, but it's very good, the battle system is also very enjoyable

I don't know if you want to include consoles, but I think a real gem is Tales of Symphonia for X360 and PS3.

EDIT: I actually meant Eternal Sonata, but both games are worth it either way.

I've got a great suggestion, it's not Japanese game though. Or do you solely want Japanese suggestions?

So...JRPG then? http://store.steampowered.com/sale/anime_sale/

if u like Beat 'em up games, i would recommend the Blazblue series.

One of my best Wishlist Anime Game, not a JRPG game, but still fresh and cool as I watch its gameplay on youtube, only 1 version : AKIBA'S TRIP: Undead ＆ Undressed

Otherwise, here is my other wishlist, just check them all, your wont regret ^^

Cladun X2
Danganronpa: Trigger Happy Havoc series
Disgaea PC
DRAGON QUEST HEROES™ Slime Edition
Fairy Fencer F
Forward to the Sky
Mighty No. 9
Mugen Souls
SAMURAI WARRIORS 4-II
Way of the Samurai series

Good timing just as the steam anime sale happens!
http://store.steampowered.com/sale/anime_sale/

Dragonball Xenoverse and One Piece Pirate Warriors 3

Dragonball Xenoverse is pretty awesome
Danganronpa is awesome as well

They've been mentioned already by others but I just have to mention them too! :D

For quirky humor and a nice (mostly) cheerful story can never go wrong with Recettear or the Hyperdimension Neptunia series (especially the 3-rd one, but I'd recommend playing it after playing the others, even though the second one kinda sucked).
Since you specifically asked for something you "can sink in hundreds of hours into" then I feel safe in recommending Mugen Souls cuz it would literally take hundreds of hours if you decide to go for 100% completion. Even I haven't mustered the courage (and time) to do so yet. O_o
While not as long as the other games I mentioned, Valkyria Chronicles would definitely surprise you if you expect a typical JRPG (because it by all means is not a JRPG, I don't call it "over-the-shoulder XCOM" for nothing).

While it's not an anime game, Might & Magic: Clash of Heroes has a similar(-ish) art style and would be the game I'd recommend for very interesting and unique game mechanics (also don't expect hundreds of hours here, it's much a bit shorter).
... How about 100% Orange Juice. Though I should mention that that game is 99.(9)% RNG.

And that's about it from me for now. I could probably recommend many many more games, some of them even anime-like, but the other anime games are quite a bit shorter and more action focused

http://store.steampowered.com/app/214830/
http://store.steampowered.com/app/240970/

play Sakura or Fault Milestone