I would guess Star Trek is the longest running, multi-generational, ever popular, science fiction series in the United States. Star Wars is a close second. I don't have any stats to back this up, just my own observations taken from multiple visits to science fiction conventions and my knack for attracting fanboys. Batman would probably edge Superman out as all time best superhero, though Marvel has done well with Spider-man and the X-Men.
I've enjoyed all these fantastic tales, but they just don't measure up to The Doctor. He is a science fiction superhero. In the fandom of Doctor Who, I'm a relatively a newcomer. Handicapped by not being British and sporadic delivery of past episodes (mostly from the Tom Baker era), I didn't really get into the Time Lord's world until the BBC revived the series and cast Christopher Eccleston as the Ninth Doctor and the SciFi channel picked up the series a few years ago.
Eccleston's Doctor rather creeped me out. He was an ominous and stern Doctor, but his companion, Rose Tyler, balanced out the duo. Rose (played by Billie Tyler) was a gem in the rough. Undereducated, unmotivated, and unemployed - but not stupid- and so much more than a pretty sidekick. Another hook that got me into watching was the brilliant writing and fantastic effects. Not often do we get the combination of writing with visual effects that don't overshadow the story (or are used instead of a story.)
Eccleston opted out after a season, but since Time Lords regenerate, the show didn't die. Instead, they find a new Doctor and each one is a unique person, but with the memories and knowledge of all the incarnations.
David Tennant came aboard the TARDIS as the 10th Doctor. Rose stayed on. There was great chemistry between the 10th Doctor and Rose. Tennant's Doctor was cheeky, smart, curious, and so much fun! I became addicted to the show watching it on the Sci Fi channel and BBC America over and over. The storyline featuring the Doctor and Rose was compelling and heartbreaking at times. The next companion, the more serious and studious Martha Jones (played by Freema Agyeman) was more intellectual and haunted by the memory of Rose. The third companion, Donna Noble (played by Catherine Tate) has been more comedic and played on the Doctor's need of a companion to keep him in touch with his emotions.
Last fall, Tennant announced he would not be returning to Doctor Who. He has been massively popular as The Doctor and a major reason for the success of a full-blown revival of the series which actually began in 1963. I was crushed. The 10th Doctor is my favorite sci fi character of all time, even though the spin-off series, Torchwood has the dead sexy Jack Harkness (John Barrowman) leading the team on that show.
So knowing that there are just a few special episodes left for the tenth Doctor (and it'll take the next two years to get them here in the US), I've been a watching the speculation of who will be cast as the 11th Doctor. Would he still be a white guy? Young or old? Female, perhaps?
The news came out today. The 11th Doctor has been chosen. He will be played by Matt Smith, the youngest actor to win the role. I stumble onto an interview with this actor discussing his new job and he seems to be properly excited about it. That was a key reason David Tennant did so fantastically well in the role- he had gotten into acting with the dream of one day playing Doctor Who so he threw himself into it enthusiastically. His love of the character came through the screen.
Now those of you who are hyped on the new Star Trek prequel coming out may scoff at my news of a new Doctor. That's fine. I'm aware Doctor Who is not as popular in the US as Captain Kirk. I'm tolerant of Trekkies, Klingons, and Next Gens and all that. But I'll take a trip in the TARDIS, though it ain't much to look at, any day over an adventure on the Enterprise. After all, the TARDIS can go through time AND space, and doesn't need a holodeck to do so.
(TARDIS = Time and Relative Dimension in Space)