It's been almost six decades now since the very first episode of Doctor Who first aired. It hasn't always been plain sailing, but, after more than 850 episodes, the show still has a fairly large following and fans all over the world. A lot of the series' themes and antagonists have remained largely unchanged during this time, although the same cannot be said of its titular character.

RELATED: 10 Sci-Fi Hidden Gems Everyone Missed

Since the show's debut back in 1963, there have been thirteen different iterations of the Doctor; 15, if John Hurt's War Doctor and Jo Martin's Fugitive Doctor are included. Each one has brought something a little different to the role, although some have done a much better job of bringing the classic sci-fi character to life than others.

10 The Third Doctor (Jon Pertwee)

Though they are all a little different, no Doctor stands out quite as much as Jon Pertwee's. Those that arrived before him and many that have followed since have leaned into the idea of brains over brawn. The third Doctor, however, liked to use both in equal measure and was a keen practitioner of Venusian aikido.

The character's physical prowess completely changed the vibe of the show at times, although his personality was not too different from those of his predecessors. That said, his anger and frustration did sometimes get the better of him. The storylines in which he featured weren't all that great, although fan favorite characters like Sarah Jane and the Master were introduced during Pertwee's tenure.

9 The First Doctor (William Hartnell)

It's been more than half a century since William Hartnell's Doctor made his on screen debut and while he may not be the most memorable Doctor, he just might be the most important. Against all of the odds, Hartnell helped the show to build an audience and laid the foundations onto which much of its success has since been built.

Hartnell's was a cold and distant Doctor that tended to talk at people rather than to them. What he lacked in personal skills though, he more than made up for with his grandeur and credibility. Future Doctors would go on to fill in some of the gaps, but without that credibility, the show would likely have been canned after just one season.

8 The Eleventh Doctor (Matt Smith)

Matt Smith's Doctor borrows a lot from the character's fourth iteration, but while both offer a healthy dose of quirkiness, the former arguably does so at the expense of the character's dignity. During some of the lighter story arcs this isn't really an issue, but, when the stakes are high, it can sometimes be a little difficult to take the eleventh Doctor too seriously.

RELATED: The 10 Hardest Sci-Fi Games Ever Made

Following in the footsteps of David Tennant was always going to be difficult, but Smith's Doctor falls some way short. He does have his moments, but his zany antics are just far too often at odds with the show's dark storylines. Much like the fez and the tweed suit, Smith's interpretation of the character and Stephen Moffat's writing just did not go well together.

7 The Thirteenth Doctor (Jodie Whittaker)

The decision to replace Peter Capaldi with a female Doctor didn't go down too well with some series fans, although Jodie Whittaker has done an excellent job of silencing her critics thus far. Granted, the thirteenth Doctor's exuberant and kind-hearted personality is in stark contrast to the dour disposition of her predecessor, but it works just as well most of the time.

Whittaker certainly hasn't been helped by some of the more recent story arcs though, which many feel are missing both the bite and the bravado that fans had come to expect in the New Who era. The last few seasons have been some of the most underwhelming since the show's return in 2005, although Whittaker's strong performances at least offer fans a silver lining of sorts.

6 The Ninth Doctor (Christopher Eccleston)

Christopher Eccleston's tenure as the Doctor was a relatively short one, but the actor played a vital role in the revival of a series that, at the time, had not been broadcast regularly for more than 15 years. Rose and Jack certainly helped out with some of the heavy lifting in this regard, but the brunt of the weight was very much on the Doctor himself.

Eccleston's perhaps wasn't the most likable Doctor that fans have seen over the years, nor did he offer up the over the top campiness that many had come to expect from the character. The grit and volatility that the actor brought to the role perfectly complimented the story arcs in which he featured though and was arguably exactly what the show needed at the time.

5 The Fifth Doctor (Peter Davison)

Filling the shoes of Tom Baker was never going to be an easy task, but Peter Davison was a much better fit than some might have expected. His Doctor was defined by his charm and charisma and these are qualities that Davison was able to deliver consistently throughout his three years on the show.

RELATED: The 10 Most Realistic Sci-Fi Futures Depicted In Video Games

While the decision to revert to a more serious Doctor may have been seen as a backward step by some, Davison's tenure also brought with it the return of some of the series' most iconic antagonists. There were ups and downs, but, considering what was to follow, hindsight suggests that the actor deserves quite a bit of credit for keeping the show afloat.

4 The Second Doctor (Patrick Troughton)

It took fans a little while to warm to the idea of the Doctor regenerating, just as it took time for Patrick Troughton to grow into the role. As time passed, however, he really made it his own. The second Doctor was incredibly likable and possessed all of the qualities that one might expect to find in a time-travelling hero. Best of all though, he was a lot of fun.

Where the first Doctor was stern and solemn, Troughton's was kind and caring, which made for a far more relatable character. He still had plenty of larger than life moments, but these were expertly balanced out by some wonderfully personable interactions. As a result, he felt like an enthusiastic teacher rather than just another apathetic space cop.

3 The Twelfth Doctor (Peter Capaldi)

Peter Capaldi's Doctor couldn't have been more different from the one that he replaced and, to many, this came as a breath of fresh air. His stern and serious demeanor may not have been for everyone, but it did provide the character with an air of gravitas that his predecessor just never quite managed to find.

Much like Smith's tenure as the Doctor, there were highs and lows when it came to Capaldi's time with the show. "The Doctor Falls" and "World Enough and Time" serve as two of the former, but the twelfth doctor's finest hour undoubtedly came in the penultimate episode of the ninth series, "Heaven Sent."

2 The Fourth Doctor (Tom Baker)

Tom Baker's seven year tenure as the Doctor is longer than any other actor's; before or since. His Doctor was fairly unorthodox for the era, yet the combination of strange and solemn worked surprisingly well. This in turn led to a noticeable shift in tone for the show; the effects of which can still be seen in modern day episodes.

With his iconic woolly scarf and wide-brimmed hat, the fourth Doctor embarked upon countless memorable adventures. Some were weird and wacky, while others were a little more serious, but Baker's performances were consistently assured regardless of what was being asked of him. That Sarah Jane tagged along for many of those escapades made them all the more enjoyable.

1 The Tenth Doctor (David Tennant)

For decades, the show has shifted between zany and serious Doctors at fairly regular intervals, but never before has an actor been able to provide such a perfect balance of both. Such is the range and brilliance of David Tennant, however, that he was able to do so consistently for five glorious years.

The story arcs containing the tenth Doctor are also some of the very best to have featured in the modern era of Doctor Who and his relationship with Rose adds a lot more depth to many of them. In the years that have followed Tennant's departure from the show, no Doctor has even come close to comparing, nor is one likely to in the immediate future.

NEXT: 10 British Crime Drama Series To Watch If You Like NCIS