J-Pop is most known around the world by anime fans, who have the chance to listen it in the openings and endings of the series. Some songs fits so well with the series that remain nostalgic for the fans, and there are some openings or endings themes that become immortals in our heads.
The animes who were most transmited on western television, and therefore have an considerable amount of fans, usually had songs that captivate the ones that hear them over the childhood.

Rurouni Kenshin's ''Tactics'', InuYasha's ''My Will'', Naruto's ''Haruka Kanata'' and Evangelion's ''Cruel Angel Thesis are just one of the famous exemples. But the number of beautiful songs in anime soundtracks is endless.

My all-time favorites recently are:

1. Kozue Takada - Secret Base (EUREKA SEVEN 1st Ending)
2. You Izumi - Dame! (RUROUNI KENSHIN Last Ending)
3. TomatoCube - Watashi Ga Iru Yo (ONE PIECE 3rd Ending)
4. Nobodyknows - Hero's Come Back (NARUTO SHIPPUDDEN Opening)
5. Mai Yamane [feat. Seatbelts] - The Real Folk Blues (COWBOY BEBOP Ending)
6. Mr. Big - Shine (HELLSING Ending)
7. Maki Otsuki - Memories (ONE PIECE 1st Ending)
8. Maaya Sakamoto - Gravity (WOLF'S RAIN Ending)
9. Siam Shade - Junjou Na Kanjou (RUROUNI KENSHIN Ending)
10. Shuntaro Okino - Cloud Age Symphony (LAST EXILE Opening)

This might not count as j-pop, but Nightmare - Alumina, to me, is synonymous with the intro/outro of Death Note episodes.

Italy has its own totally sucky opening/ending soundtracks. I only know some Naruto's cause I watched some episodes in Japanese with sub. Yeah, Haruka Kanata is WIN.

I may hate the anime and the manga in the core of my guts, but the soundtrack of the anime Bleach reunites some nice collection of J-Pop and J-Rock.



This first one is mostly the original soundtrack of the series landscape and action scenes, but it has some of the early opening and ending sequences.



Now with the real deal, nostalgic and great pop-rock from Japan... when Bleach was still good XD

Dunno if call it a guilty pleasure, but I do really dig some Anime j-rock/pop soundtracks. I like some mexican/latin-american versions of some anime openings, some of them are pretty good(at least for me, lol, sadly, nowadays they aren't as good as they were before). Some of my favorites right now are:

Maaya Sakamoto - Yakusoku Wa Iranai (The Vision of Escaflowne Opening)
Lia - Tori No Uta (Air Opening)
Chemistry - Period (Full Metal Alchemist Brotherhood 4th Opening)(I haven't watched the series yet)

Quote:
10. Shuntaro Okino - Cloud Age Symphony (LAST EXILE Opening)

This one's good, but I liked better the BGM used in the series.

Yeah, it's a bit of a guilty pleasure of mine too but I really like:
Aki Toyosaki- Utauyo!! Miracle(K-on!!)
Claris- Irony(Oreimo)
Aya Hirano- Super Driver(Suzumiya Haruhi No Yuutsu)

Hmmm yeah there are some really freking good intro/outros out there.

Pretty much all the itros/outros of Rurouni Kenshin imo.
Neon Genesis Evangelion, awesome intro as well.
Elfen Lied has one of the most beautiful intros ever, that shit maked me cry one time, it's just that nice.

My favorite at the moment has to be the intro to Hunter X Hunter from the Greed Island OVAS.
I don't know... It's just so catchy

I don't really remember the name of the songs but I know which Anime OSt's I enjoy.

Death Note's intro/outro and the whole OST tbh.
FLCL's OST, watching this anime is an experience in itself.
Berserk and Neon Genesis Evangelion's OST.

These are from the top of my mind.

Well, this is more J-Rock, but Kenji Ōtsuki makes some good opening and ending themes. he also made Akuma Stocking for Hellsing Ultimate

In no particular order:
Naruto - Yellow moon, Long kiss goodbye, Hero's come back, Haruka kanata, For you, Yura yura, Broken youth
Nana - A little pain, Winter sleep, Wish
Bleach - Tonight, Thank you, Asterisk
Fruits Basket - Small prayer, For fruit basket
Full Metal Alchemist - Melissa, Rewrite, Undo, Ready steady go
Death Note - The world, Alumina
Toradora - Vanilla salt
Ranma 1/2 - Don't make me wild like you, Little date, Don't mind lay-lay boy, So many memories
Sailor Moon - Moonlight densetsu, Heart moving
Wolf's Rain - Gravity
Air - Tori no uta
Slam dunk - I want to shout I love you
Nodame Cantabile - Allegro cantabile sound, Konna ni chikaku de


...oh yes!! I'm really into them