Stein’s Gate’s OP with the song Hacking to the Gate is probably my favorite overall including both visuals and the song itself.

Prior to watching Stein’s Gate my favorite was Shakugan no Shana’s first OP with the song Scarlet Color Sky.

I also really liked the OP for Mahou Tsukai ni Taisetsu na Koto (Someday’s Dreamer) with the song Kaze no Hana which was my favorite OP before that.

Honorable mention to Honey and Clover for having the most WTF OP out of all the anime I have ever seen.

If you’re looking for something else to listen to, basically any OP by Yoko Kanno or performed by Kalafina is good. e.g.:

• Hikari no Senritsu from Sora no Woto
• Tank! from Cowboy Bebob
• to the beginning from Fate/Zero (EDIT: I originally said “both OPs” but I got mixed up with one of the EDs – the other OP is oath sign performed by LiSA)
• the Ghost in the Shell SAC OPs – particularly Rise fron 2nd Gig

A few others that I enjoyed are:

• Asterisk – Bleach’s first OP
• Only My Railgun – A Certain Scientific Railgun
• Choir Jail – Dusk Maiden of Amnesia

but there are way too many to list, so I’m going to stop there before I write a novel.

Edit: Added links. Thanks @Davel23 – I’ve been trying to stay off of YouTube lately and didn’t know about that site until you posted your comment!