I agree that series four had the best intro; thankfully a change from the in-your-face music from the third series. All the characters got their little bit and it showed off the eclectic nature of the show in general.
A lot of people seem to think series five was the best, but I have two problems with it. Although the overview is nice, the music is simply the end credit music from series past. And secondly, the billboard proclaiming who created it (as if we didn't know) on the back of the station was the sort of ego trip you'd expect from... well, someone named Gene Roddenberry.