I love WataMote's OP myself too. I also love Rising Hope too - ugh, can't stop listening to that OP.
Like GabbyHeart and GeneralOtter, I have a lot myself too. Too many to point out. In terms of instrumental tracks, I mostly dig Madoka Magica's soundtrack though. Yuki Kajiura is a master.
On the non-instrumental side, I actually made a whole list of my favorite OPs and EDs on my MAL profile:
Opening
A Certain Scientific Railgun
Black Lagoon
Blue Exorcist
Bokurano
DanganRonpa: Academy of Hope and High School Students of Despair - The Animation
Darker Than Black: The Black Contractor
HOWLING
Deadman Wonderland
Digimon Tamers
Fate/Stay Night ~Realta Nua~
,
Fullmetal Alchemist: Brotherhood
, Rain
Ga-Rei Zero
Gintama
,
Highschool of the Dead
Higurashi no Naku Koro ni
Naraku no Hana,
,
Hourou Musuko
Kino's Journey
Mahouka Koukou no Rettousei
Rising Hope
Mirai Nikki
Nichijou
,
One Piece
Paranoia Agent
Pocket Monsters
Mezase Pokémon Master, OK!, Ready Go!, Advance Adventure,
,
, Together
Pokémon (English Dub)
Rewrite
Shiki
,
The Melancholy of Haruhi Suzumiya
,
WataMote
Ending
Bokurano
,
Detective Conan
, Secret of My Heart, Summer Memories,
, Your Best Friend, Kimi e no Uso
Digimon Tamers
Fairy Tail
Gintama
Genjou Destruction
Higurashi no Naku Koro ni
,
Hourou Musuko
K-On
Don't say "lazy", NO, Thank You!
Little Busters!
Mirai Nikki
Paranoia Agent
Pocket Monsters
, Type: Wild, Maemuki Rocket-dan!, Polka O Dolka,
The Melancholy of Haruhi Suzumiya
Welcome to the NHK!
,
You can click on the links above if you wanna listen to them (most of them are dead links, I would presume), but I'm too lazy to post all of their YouTube clips, and I don't want to spam the page with many videos, so I'm just gonna post some here: