some of my favorites are:
Scandal
Maximum the Hormone
Ellegarden
Asian Kung-Fu Generation
Stance Punks
Alice Nine
Cromagnons
Blue Hearts
Def Tech
Hi-Standard
Beat Crusaders
kyary Pamyu pamyu
One OK Rock
Nightmare
Balzak
Nozomi Sasaki
Orange Range

Well I have a lot, I'll shorten the list to my top 7:

SCANDAL
Maximum the Hormone
the GazettE
Eriko Imai
Orange Range
Nightmare
Aqua Timez

no particular order:
SCANDAL
Ayaka
LiSA
Stereopony
Radwimps
Asian kung fu generation
Aqua Timez

SCANDAL
One Ok Rock
Alice Nine
SuG
Exist Trace
Coldrain
Buono!
AKB 48 (KINDA)
LM.C
Maximum the Hormone
The GazettE
Ellegarden
Fear, Loathing in Las Vegas
ViViD
D=OUT
SiM
Rottengraffty
SID
SPYAIR
YUI
9mm Parabellum Bullet
whew and it keeps growing...

the GazettE
SCANDAL(duh!)
Stereopony
Sekai no Owari
Tokyo Jihen
ONE OK ROCK
BIGMAMA
School Food Punishment
SKE48
C_ute
Buono!
Kikkawa Yuu
Idoling!!!
Perfume
High and Mighty Color
Applicat Spectra
Weaver
Fear and Loathing in Las Vegas
Galileo Galilei
THE KIDDIE
SuG
supercell
miwa
RADWIMPS

Yeah that's a lot .happy.

Well besides the obvious number 1 that is Scandal, heres a list of artists Ive been listening to in no specific order:

Utada Hikaru (pretty much since Kingdom Hearts)

Lisa (thanks to fellow SH posters in the music forum)

Sambomaster (favorite Naruto opening #5)

High and Mighty Color (this band looked like the japanese version of spanish band La Oreja de Van Gogh; too bad they broke up)

AKB48 (all credit goes to Mr. Tony Miller lol)

Kyary Pamyu (found her by random chance)

Kanon Wakeshima (gotta be my second favorite after Scandal)

Emiri Miyamoto (dont even ask me how I found her lol)

Besides those Ive seen other bands/artists, but they may only have one song I like, so they are one hit wonders for me.

Well, before SCANDAL there was:

The Black Mages (Nobuo Uematsu AND Final Fantasy...'nuff said)
Nujabes (such lovely chill out music...RIP)
Dragon Ash (Shizuka na Hibi no Kaidan Wo ftw!)
Hikki (just love her voice. Passion/Sanctuary and Simple & Clean, among many other songs...)
Maaya Sakamoto (her voice is tied into so many games/anime that I like. "Hemisphere" from Rahxephon is a fave)

And ummm, does Leah Dizon count?

For me SCANDAL share number one with Asian KungFu Generation. Love those bands, even though much of the lyrics is kinda lost on me. [You must be registered and logged in to see this image.]

SCANDAL 1st ( Of cuz ! xD )
miwa ( 2nd ) =3=
YUI ( 3rd )
snow 4th

-SCANDAL
-FLOWER
-Dream
-Happiness
-Amuro Namie
-YUI
-Kalafina
-Kanon Wakeshima
-Shintani Ryoko
-Aki Misato
-AKB48 (sometimes)
-Morning Musume
-Changin' My Life (they sang songs for the anime Full Moon wo Sagashite, but they disbanded already.)

Here is mine...
SCANDAL
Stereopony
Arashi
Aqua Timez
YUI
Shoko Nakagawa
HIGH & MIGHTY COLOR
Uverworld
NICO touches the walls
SID

i guess its kinda obvious that scandal would be one of them .lol.
so heres my list, in no particular order

-SCANDAL
-Yui
-High and Mighty Color
-L'arc en ciel
-TM Revolution
-Asian Kung-fu Generation
-Ai Otsuka
-GRANRODEO
-Aqua Timez
-fripSide
-ClariS
-CHERRYBLOSSOM

top 10 would be..

1.YUI
2.YUI
3.YUI
4.YUI
5.YUI
6.YUI
7YUI
8.YUI
9.SCANDAL
10.MIWA

SCANDAL [You must be registered and logged in to see this image.]

Chatmonchy

Asian Kung-Fu Generation

Jinn

ONE OK ROCK

YUI

ELLEGARDEN

UVERWORLD

Ikimono Gakari

The Brilliant Green

TommyHeavenly6/TommyFebruary6

EDIT:

Added:
Nico Touches the Wall

SID

Chemistry

Do As Infinity

Not sure if I've already posted in here, but I'll post anyway:

Buono! (The greatest thing to happen to the music industry, ever)
High and Mighty Color (The second greatest thing to happen to the music industry, Especially w/ Halca)
2side1BRAIN (ex. HaMC MEG's new band)
9mm Parabellum Bullet
SiM
Galneryus
Supercell
Doping Panda
Capsule
Nana Kitade
Kyary Pamyu Pamyu

only 2 japanese bands and they are:

1. Hey! Say! JUMP[You must be registered and logged in to see this image.] [their Dancing plus their Singing skills are really great!]

and of course

2. SCANDAL [You must be registered and logged in to see this image.] [they're definitely awesome]

My favorite Japanese bands/artists are:
SCANDAL
YUI
Hokago Tea Time/K-ON
LiSA
Girls Dead Monster (LiSA's band for Angel Beats! :D)
Aqua Timez
High and Mighty Color
Winnie
AKB48
Aya Hirano
Zone
Nujabes
Shoji Meguro (Persona! ^_^)
Hirofumi Sasaki
Kozo Suganuma
Senri Kawaguchi (One of the most amazing drummers I've ever seen for her age! :D)
Back-On
Orange Range
Mika Nakashima (You might know her as NANA :D)


This is it... for now? lol I know I'll be discovering more bands soon! :D

1. Last Aliance
2. l'arc en ciel
3. Judy and Mary
4. MOVE
5. Kids Alive
6. Rikki
7. Get Over
8. BAAD
9. SCANDAL
10. Asian Kung-Fu Generation

[You must be registered and logged in to see this image.]

My Top 10 list:
1.SCANDAL
2.Fear,and loathing in las vegas
3.The Gazzette
4.High and mighty color
5.Aqua Timez
6.Flow
7.Alice Nine
8.Vivid
9.Deluhi
10.YUI

Scandal
Scandal
My Eggplant Died Yesterday - Beast dudes .cool.
Neko
Lunetia
Shuhei
Hanatan
Wotamin
Osamuraisan - guitar beast .cool.

they all from niconico .hail.

I wrote the name of the band, the genre of music and a few examples of songs (not in any particular order, except SCANDAL is always at top )

1. SCANDAL
2. Ellegarden (Pop punk - mostly English)
(Fire Cracker, Koukasen, Marry Me)
3. One Ok Rock (pop rock/ emo)
(Jibun Rock, Re:make, No Scared, Kimishidai Ressha)
4. Stance Punks (Punk rock, similar to Blue hearts)
(I Wanna Be, The World is Mine, Sharol wa Blue)
5. Blue Hearts ('80s Punk rock)
(Linda Linda, Owaranai Uta, Hito Ni Yasashiku)
6. Asian Kung-Fu Generation (alternative rock)
(Haruka Kanata, Rewrite, After Dark)
7. Beat Crusaders (pop punk/ pop rock/ power pop/ indie rock - mostly English)
(Feel, Hit in the USA, Moon on the Water, Situation)
8. Maximum the Hormone (nu metal/ hardcore punk/ alternative/ metal)
(Koi no Mega Lover, Kuso Breakin' Nou Breakin' Lily, Minoreba Rock)
9. Cro Magnons (punk rock)
(8 Beat)
10. Yum! Yum! Orange (ska)
(Precious Days, Clover)
11. Ore Ska Band (ska)
(Hana no Ska Dance)
12. Hi-Standard (pop punk)
(The Kids are Alright, This is Love)
13.Kyary Pamyu Pamyu (pop)
(Tsukema Tsukeru, pon pon pon)
14. Nozomi Sasaki (pop)
(Kamu to Funyan, papepipu papipepu papepipupo)
15. Kemuri (ska)
(Jojou no Uta, Hana Ga Saku Toki)
16. Balzac (horror punk - mostly English)
(Inside my Eyes)
17. Tsuru (afro-rock)
(Yoru wo Koete)
18. Good 4 Nothing (Pop punk)
(It's My Paradise)
15. Orange Range (alternative rock/ rap rock)
(*Asterisk*, O2)
16. Eiji Wentz (pop rock/ indie rock)
(Awaking Emotion 8/5)
17. Alice Nine (visual kei/ alternative rock/ pop rock)
(Subaru, Tsubasa, Q.)
18. Nightmare (visual kei/ alternative rock/ pop rock)
(The World, Alumina)

Ayaka
UVERworld
YUI
SCANDAL
Happiness
FLOWER
Dream
.....
....
haha there are so many i can't really remember....

SCANDAL
YUI
One Ok Rock
T.M. Revolution
Abingdon Boys School (side project of Nishikawa-san :O)

occasionally listen to these artists:
Aqua Timez
Uverworld
Stereopony

don't know a lot of japanese artists, i only know artists who made songs in the anime...

here's my list:
1-Nagi Yanagi(atm)
2-supercell
3-SNSD (they've debuted in Japan now so i might as well list it :D)
3-SCANDAL
5-Yousei Teikoku (gothic music <3)
6-Miku Hatsune dont care if she's not human, i like her songs
7-Yui
8-ClariS
9-Flow
10-Aqua Timez
11-Chieko Kawabe
12-Yui Makino
13-Maximum the Hormone

i put snsd before scandal because ive become a sone before becoming a scandal fan but i love them equally .love.

Wow where to start well obviously

- Scandal
- UverWorld
- Utada Hikaru
- SunSet Swish
- Kelun
- Aqua Timez
- Azusa
- Supercell
- ClariS
- Hamasaki Ayumi
- Beat Crusaders
- Rei Fu
- Miwa
- Miz
- Himemiya Milan
- Chocolate Rockers
- Kitamura Eri