Best OP-ED
 

What's your most favourite OP-ED
For me they're from Eureka seven
OP : Sakura (OP4)
It's incredibly mixing between J-pop and Choir. It's pretty weird at first but as I watch this anime I love it more and more. :D
ED : Himitsu kichi (ED1)
It's beautiful song which really suit the sunset theme. It's very good story telling too. I love it's ease feeling and never get tire of watching it.:)

Hmm it's pretty hard to narrow it down to just one, but I'll try. ;_;

OP: Ghost in the Shell - Inner Universe
GitS is one of my favorite shows and Yoko Kanno is awesome, so what can you expect~

ED: Naruto - Wind
I know it gets a lot of flack for being engrishy, but I really like the mood of the song. It also reminds me of when I actually liked the show/manga. =P

Though I haven't watched much anime, I have quite a few anime OP/EDs from playing Stepmania way too much.

Personally my opening favorite is "For Real" by Hidenori Tokuyama from Saiyuki. I've never watched the anime. I think the intorduction of the song is sampled from an Aerosmith song "Eat the Rich" I believe.

And for closing "Kujikenai Kara!" by Masami Okui and Megumi Hayashibara from Slayers. I dunno there's just something to it that makes it stay in my head for days after I listen to it. Also never seen the anime (but want to).

Romeo X Juliet

OP
Lena Park - You Raise Me Up

Toward the Terra

OP
Endscape - Uverworld

ED
Miliya Kato - Love Is...

GAWD, it's hard to narrow it down to just one because I have so many favorites. But in the end this is what it came down to.

OP: Chrno Crusade - Tsubasa wa Pleasure Line
ED: Naruto - Wind

Hmm...

OP: Mousouka (from Utawarerumono)

<3 Suara. That and it's the first anime I watched. I've watched plenty more afterwards, but I liked this opening song a lot.

ED: Kimi ga Tame (from Utawarerumono, ED2)

Same reason as above. This barely qualifies as an ED, since it's played only in Episode 26 as the series as a whole is ending. But this song is amazing. But if you don't count that...

Real ED: Hare Hare Yukai (from Suzumiya Haruhi no Yuuutsu)

Usually, I don't like the ED songs, but this one is different. Complete with choreography too.

These will probably be always changing, especially since so many come to mind with all the different styles. The ones in my mind for the longest duration I can remember are currently:

OP: Dirty - Majin Tantei Neuro Nougami, OP
Simply a hard rock song by Nightmare. The hooks lie in the tempo changes, the usual electric guitars and percussion, and Yomi's characteristically unique voice. Easily 100+ listens for me within a short period of time.

ED: Ningyo Hime - Chobits, ED2
I've only read the manga for Chobits, but found this ending to be very melancholy, with suitable, non-random lyrics. The piano and the acoustic mix well for me here, and Rie Tanaka's voice clearly depicts the intended sense of longing.

Honourable Mention:

Cloud Age Symphony - Last Exile, OP
This is weird, although probably not as weird as Paranoia Agent's OP song. Chant-like laid back indie style with the standard quarter bass beat.