All these songs are great:
Hunter x Hunter opening 3 (Pale Ale)
Hunter x Hunter Ending 3 ( Hotaru)
Someone already said Shura from Gintama
Gintama op 4. (Kasanaru Kage) \
Gintama op 3 Yuki no Tsubasa \
Gintama ed 7 Signal
Gintama ed 8 Speed of Flow
Reborn op 4 (88)
Reborn ending 2 (One Night Star)
Inuyasha ending 5 Song of Truth
Law of Ueki op 2 (No Regret)
D.Gray-Man op 2 (Brightdown)
D.Gray-Man op 3 (Doubt & Trust)
Shippuden op 3 (Blue Bird)
Shippuuden ed 5 (Broken Youth)
Yu Yu Hakusho op 3 (Smile Bomb)(DUB)
Yu Yu Hakusho ed 5 (Daydream Generation)(DUB)
Shin chan End (DUB) (Party Party)
Code Geass Op 3 (Hitomi no Tsubasa) My favorite song
Code Geass R2 Op 1(O2)
Code Geass R2 Ed 1 (Siawase Neiro) Love this song
Bleach Op 3 Fav op from Bleach (Ichirin no Hana)
Bleach op from Soul Carnival
Actually ALL songs from Bleach are great.
Pokemon Johto League Champions is great too( I think all the PKMN op's were too good for the actual show):
Pokemon Advanced
Rurouni Kenshin op2 (1/2)
Digimon Savers Op2 Hirari
Digimon Savers Ed 2
Bobobo op 2 BAKA SURVIVOR
Bobobo ed 3 HPSJ
FMA op 4 (Rewrite
FMA ED 2 (Beyond the Door)
YGOGX OP 3 (Teardrop)
YGOGX OP 4 (Precious Time, Glory Days)
And of Course Brand New World:
and We Are! Remix
Sorry, that was alot, the ideas kept flowing in.