TOS by miles and miles. TNG has a few well-scored eps, and I think ENT had a couple too, but the oft-reused TOS cues are often amazing (the one-offs are pretty cool too, like the ELAAN OF TROYIUS stuff, which sounds like it could be VICTORY AT SEA, it so evokes WWII for me.)
This belongs in another thread that does exist...I remember a thread about which series had the best music throughout, I participated in it. This one is just about best opening credits music. I agree about the original series having the best opening theme. I used to think it was cheesy, but now I love it and enjoy watching it every time.
I always thought the TNG one was too over-the-top for the opening credits of a TV show and the narration was kinda corny. Great music, just not suitable for the opening of a TV show which I think should be faster-paced and more catchy.
DS9's theme is all right, but just sort of a more low key version of the TNG theme, which annoyed be when I first heard it, though it's since grown on me. Like the show itself, I think Voyager's theme is just a cheap knock-off of previous Star Trek. To me it sounds like a lamer version of the DS9 theme. I can't believe people actually prefer it over the others. I think it's like the previous three Star Trek themes except not as good, just like the show.
P.S. I agree about that Dallas Star Trek fan edit...hilarious AND awesome! I especially love the specific poses chosen for the characters, particularly Geordi's "coolant leak!", Data laughing his ass off, Worf, Picard, and Dr. Crusher in various costumes (I don't even recognize the uniform Crusher is wearing on the right, what the hell is that?), and Tasha looking sexy for once! They should have had one shot of Wesley in his Starfleet uniform, just for variety's sake.