If you want you don't have to say just one. You could do three that you like :)
I'll give my three :)
^^^Spoilers, kiddies!^^^(every time I get used to the One Piece intro they change it :/ )
( I apologise to all the angry weebs for putting up intros for the three most mainstream anime, but they just have the best intros :3)
Most Helpful Girl
This is going to take a little while because I have 4 lol. They're not in order or something.
1. Deadman Wonderland opening. I love the music and the video how twisted it is with the figures being controlled and stuff, because the anime self is so twisted.
2. Tokyo Ghoul opening first season. I love the artsy colours and how the characters look troubled. I also like the music a lot. This theme song is the ringtone of my phone actually.
3. Fullmetal alchemist brotherhood opening. The song is just great. Sometimes I just listen it as a real song. The video itself is very cute with the children and stuff, but also a lot of action and a bit of sadness. Perfect description of the anime I think.
4. Elfen Lied opening. This is just so beautiful. It's inspired by a famous painting called 'the kiss' and I think they made it so beautiful. It almost looks like Lucy is sort of trapped by this anonymous figure and that makes it also a bit mysterious.
Most Helpful Guy
I have two contestants:
1.) Steins;Gate OP
Fun fact: the sub I downloaded marked the chapters in each episode, and they marked the opening with "CPU Benchmark"
Fun fact 2: we were trying to watch Steins;Gate on my old laptop, but the opening is so massive that it lagged the hell out of it - the episode played fine otherwise, though! :P
2.) Future Diary OP 1