The best, singular? Going to have to go with Dimension Tripper, the opening to Hyperdimension Neptunia: The Animation. I love the song, and the entire opening sequence; the second one used in particular.
If we're allowed to pick more than one...hoo boy. Pretty much anything by Ali Project, Kalafina, and the Zetsubou Sensei cast is going to appear in my post if so.
I know I talk about Fairy Tail a lot, heck I even run the FT club on here, but that's because it's one of the few animes I've watched and it's one of 2 I've ever finished/caught up on. Overall it's my favorite anime I've watched, so of course I'm gonna have to pick from FT openings, which is really hard for me. Out of the 22 that are out, I'd say openings 1, 3, 4, 9, 13, and 18 are the best. Out of those, I'd have to narrow it down opening 3 being the best, although 4, 13, and 18 are my top 3 favorites.
Edit:
I'm starting to think 18 is better, but maybe that's just my mood. My perception of good music tends to change with my moods.
Oh, you kill me with the choice, but if I absolutely have to...
Oh man, it's been such a long time since I watched that. Though I like the Stone Cold OP more since it stuck with me all this time, while I don't remember your song at all. I don't think I remember the show either, other than MCs obsession with collecting rocks. And gg's changing everyone's name into Final Fantasy characters, so MC was sometimes Squall, sometimes Ramza. Good times.
As for my choice, I'd pick Shin Mazinger Z's first opening, which is probably my favorite Jam Project song used for anime (their Garo songs are pretty good too, and of course Super Robot Wars themes)
I'll also have to include my retro pick as well, since the 80s were pretty crazy with catchy openings
I have a few favorite openings overall, but out of the new spring anime, the one in Bungou Stray Dogs is really good in my opinion (I am only watching a few from spring, but whatever).
There's so many anime openings I love, It's really hard to choose just one.
I will say both Shingeki No Kyoujin's openings, just because Linked Horizon is a side project of the founder of Sound Horizon which is currently one of my favorite "bands" (if we can call Sound Horizon that hahaha) ^^
I was told the image in my sig was made by Ichinose Kanade ^-^