She's from the 21st Century (hey, that's ours!) with a cute little upturned nose and bob cut. That, and she's an Astrophysicist with a photographic memory and a genius level intelligence. So, a woman who not only looks good but can do vector calculus standing on her head? Be still my oscillating waveform! Zoe also has a strange and alluring sense of fashion, as shown in this clip in which she uses her knowledge of mathematics to make an evil computer blow up:
I repeat, she makes a computer blow up through math. How do you not want her?
4) Sarah Jane Smith
There's a reason that she was picked to be in a spin-off show with K-9 not once, but twice -- she's just damn appealing. Not only is she cute with her friendly, slightly crossed eyes and pursed lips, but she doesn't take any crap, while being very optimistic and friendly even aliens that look like they may fall apart at any second.
Also, consider this -- in her appearance in the episode "School Reunion," Elizabeth Sladen was 57 at the time. She sure looks good for a woman around her sixth decade. Seriously.
3) Romana I
The first Romana was beautifully elegant. She was also the first companion of the Doctor who was also a Time Lady. Who doesn't want to have sex with a Time Lady? I mean, a lady have a milkshake that brings all the boys to the yard, but a Time Lady's yard is bound to be bigger on the inside than on the outside, which makes it fit more boys. Also, again, smart is sexy and Romana was almost equally as smart as the Doctor.
Here we see a portion from "The Pirate Planet" where The Doctor fails to get the attention of the locals; around the 2:36 mark, you can see that K9 suggests to let Romana try because, simply because "she is prettier than you, master." Quite right.
2) Amy Pond
Holy fuck on a stick. She's not only a redhead and looks fantastic but she's got that Scottish brogue that gives her a bonus of +10 hotness. Though that's not really what puts her near the top of the list -- it's all of that plus the fact that she's, smart, confident and fearless. Extra bonus points for doing what most companions of the Doctor have wanted to do but have been too chicken shit to do it -- actively saying, in no uncertain terms, that she wants The Doctor and then actively going for it. Confidence in a lady is just as sexy as in a man.
1) Romana II
So Romana regenerated for reasons that don't really make sense in the series; in reality Mary Tamm was tired of being relegated to a damsel in distress despite her character being a match for the Doctor in intelligence and ability. Fair enough. In her place, we get a short ,blonde little perky Romana with an enormous smile. Some might say that the first Romana was more attractive than the second, but I think this one is just more attractive. She's got such charm to her. She smiles, you want to smile back. Also, Lalla Ward, the actress who played the character, went and married Tom Baker for a while before moving on to long-time husband Richard Dawkins after her friend Douglas Adams arranged a meeting. What kind of girl marries the Fourth Doctor and then leaves him to marry Athiest Central with the help of the Forefather of Forty-Two? Answer: THE LORD HIGH QUEEN OF NERDS.
Therefore, the second Romana gets about a billion bonus points to sexiness. Here's her above in the Adams-penned "City of Death" being as cute as ever.