Bored tonight, so let's a do a Top 5 list.
TOP 5 DOCTOR WHO COMPANIONS
(TV only - sorry folks who are fans of the novels, Big Finish audios and the like. Though if I did open it up to stories of questionable canonicity, Benny Summerfield would definitely make the list, though I've read just one and a half stories in which she is featured. She's pretty awesome.)
(Also, I haven't seen Season 3 of the new series yet, so Martha Jones is out by default. I do hear good things, though.)
Honorable Mention: Brigadier Allistair Gordon Lethbridge-Stuart. He only ever made the odd short hop in the TARDIS, so he's not a companion in the traditional sense, but his frequent reappearances (in all the Who-related media) have made him such an important figure in the show's mythology and fleshed out his character so much (even retired, the man kicked ass) that he belongs on the list in some fashion regardless.
5.) K9. He's a dog. He's a robot. He has a ray gun in his nose. He has a bit of an ego. What's not to love? Plus, he was far more versatile - and significantly less annoying - than the Doctor's other robot companion, Kamelion (who only appeared twice, to my recollection: his debut, and the episode where he gets blowed up real good. Am I missing any?).
4.) (tie) Leela and Ace. I always rank them together, since they're variations on the same theme: women with untapped/overlooked potential who approach every situation with a similar question: how can I stab this (Leela) / blow it up (Ace)? And that's always fun. Plus, seeing the Doctor(s) play Henry Higgins to their respective Eliza Doolittles always made for some fun situations, though the really fun stuff happened whenever the Doctor actually let them cut loose ("Ace, hand me some of that Nitro-9 you're not carrying.").
3.) Post-Regeneration Romana (the Lalla Ward version). Usually the Doctor plays the caretaker / tour guide role with his companions. But Romana, being a fellow Time Lord (okay, Time Lady), was the first companion since granddaughter Susan to be on anything approaching equal footing with him. Sure, the Doctor was the experienced traveler helping her break free of her staid Gallifreyan existence, but she was his intellectual equal (and in some respects, probably his superior), making for an interesting twist in the Doctor / companion relationship. And I prefer the second Romana because a.) she's now been around for awhile and is starting to really dig the ride, and b.) I happen to find Lalla Ward more attractive than Mary Tamm.
2.) Jamie McCrimmon. The Doctor tends to prefer traveling with cute women rather than guys (well, wouldn't you?), but there have been a few males here and there. The problem is, a lot of 'em were complete tools (Harry Sullivan? Imbecile.). The Brig was good, Ian Chesterson seemed like an alright guy, and Captain Jack is undeniably cool, but my favorite of the guys was always Jamie. Just about everyone else the Doctor has ever traveled with was definitely a companion. Jamie was more akin to being the second Doctor's buddy... lots of joking and mock adversarial banter. The sort of thing a couple of guys would do, really. Not the sort of relationship we see on this show very often. Plus, he got to travel with both Victoria and Zoe, two companions who might not be my favorite overall characters, but would definitely make my "Companions Who Have Teh Hawtness" list if I were to make one. Lucky man.
1.) Sarah Jane Smith. I think we can agree that Sarah Jane is pretty much the ultimate Who companion. Smart, tenacious, decent in a fight, willing to go along with the Doctor but unwilling to take any of his crap, screams well when the situation requires it, and best of all, I don't recall her ever stupidly twisting her ankle at inopportune moments (I always hated that old Doctor Who chestnut). And it certainly doesn't hurt that she was pretty cute. It's never been particularly surprising to me that of all the former companions, she's the one that's had the most shelflife after being written off the show: brought back for an anniversary special, brought back for the new series (only one so far for that), her own range of Big Finish audios, and two TV spin-offs. So clearly I'm not alone in my admiration here.