That is a very difficult question, because while I generally prefer ending themes to openings, there ARE a lot of good openings. My favorites, in no particular order:
1) I really like
Fate/Zero's second opening (
http://www.youtube.com/watch?v=5FznFkKlKio). I like Oath Sign too, but I do think the second OP is slightly better choreographed.
2) For comedies, I think
Gintama's second opening is the best (
http://www.youtube.com/watch?v=jbv5rCcTCfc).
3)
Sword Art Online's first opening, Crossing Field, just oozed potential and made it seem like a fascinating adventure (
http://www.youtube.com/watch?v=Yg98e4twWmo).
4) The opening for
Serial Experiments Lain needs to be on here. I can't justify leaving it out, even for variety's sake (
http://www.youtube.com/watch?v=t9CXmEUwvgM).
5)
Jojo's second opening Bloody Stream was really good. I haven't watched enough to say it's the best opening, but I like it (
http://www.youtube.com/watch?v=D-HWQ4hQPo8).
For the sake of variety,
6)
Mondaiji-tachi had a great song and got you pumped up each episode (
http://www.youtube.com/watch?v=ySn55tUVfxM).
7)
Eden of the East had a strange, yet satisfying opening (
http://www.youtube.com/watch?v=4pdqeu9LW2c)/
8)
Penguindrum's second opening had a good song and some nice imagery (
http://www.youtube.com/watch?v=NB2sow7tbJ8). The first opening was good too, but I think this one is slightly better.
9) I agree with
This Ugly, Yet Beautiful World's opening (
http://www.youtube.com/watch?v=EgXcmvfGsYU). The anime wasn't that bad, actually, but not amazing either. The opening made the show.
10)
Kiba's opening was probably the only reason I kept watching it. And I still never finished (
http://www.youtube.com/watch?v=Q0_sXfTYunk).
11)
Mahoromatic's opening, which apparently no one has with good video quality, so a random opening that is also good (
http://www.youtube.com/watch?v=7CiaAFVeTsE).
12)
Anohana's opening. I still think the script was lacking, but I have to admit the music in Anohana was just right (
http://www.youtube.com/watch?v=ANJTGxUq9gs).
13)
Kyousougiga's opening is so innocent and hopeful. It also uses bright colors really well (
http://www.youtube.com/watch?v=oYDKDrQaJ9Q).
14)
Yoru no Yatterman's opening also did a great job capturing a sense of adventure. Haven't finished the anime, but I don't think it will ever reach the quality of its first episode. Like Kotoura-san.
15)
Soul Eater's second opening. I actually like it more than the first one, because I like the song more and the animation sets a darker mood for the series (
http://www.youtube.com/watch?v=obL44Ljv2VQ).
The guy that ate your pie.