Although Usagi needs all her girls, if she has to go with one with the longest time with one, I think she should definitely pick Mako.
Mako and Usagi have enough similarities (love of girly things, strong moral code), but enough differences (Mako is more ready to jump into action, Usagi is more ready to forgive, Mako likes to cook, Usagi likes to eat) to allow them to work together. It's no accident that Mako was the one in the Nehellenia Stars arc who had to get Usagi to snap out of it. The only flaw is that Mako does believe in Usagi a little too much; I'm not sure she would be able to step up to contradict Usagi if she were wrong.
As for the other choices:
Ami - Ami's a good teammate and supportive figure, but she's also the tactician. While she can help in battle, she can't do that and protect Sailor Moon, and Sailor Moon isn't exactly a defensive specialist. Also, Ami, at least in the anime, is a little too easy to fold without Usagi giving in the same way (for instance, Usagi gets Ami to loosen up, but by contrast we never see Usagi try harder in school as a result of Ami's influence). This is a problem when both of them are feely-types at heart; even when Ami is rational, she's never emotionless or truly cold.
Rei - For entertainment, this would be comedy gold, but realistically, the two of them would butt heads too much if forced together for long without anyone breaking them up. They could have great teamwork when they're on, but when they're off, they will argue and disagree, and without any mediating figures, each of them would go their own way because they're that stubborn. With only two of them, they'd split.
Minako - They balance each other out well in battle, but both of them are prone to fits of silliness outside of battle and would only end up exasperating each other's bad habits.
As
@Star Angel Haruki says, the anime gives a good reason why Haruka would be an awful teammate and the same is true of Michiru. Both of them have a fundamental difference in worldview from Usagi and are too prone to go their own way. They'll work well with each other as well as with a kindred spirit who will share their ideals, but they are frustratingly isolationist.
Setsuna would be a disaster, even discounting the manga attraction she has to Endymion. The biggest problem is the mental age difference, as Pluto would be working from what would basically be an infant in her view.
Hotaru is an interesting case, as they are complete opposites in terms of personality. That being said, it's often forgotten that while Chibi-Usa had a special bond with Hotaru, that Usagi herself had a similar, but different bond in S beyond being Chibi-Usa's tagalong pal. In civilian form, both could teach each other a lot (Usagi could expose Hotaru to life, and Hotaru could show Usagi), while their divergent approaches to evil would compliment each other. Plus, in battle, Saturn's wide array of powers (both offensive and defensive) give her more flexibility than one would expect. However, that whole "going my own way" stuff the Outers are prone to do will rear its ugly head, and Saturn's tendency to consider herself a last resort and an uninvited guest could make things difficult. Saturnine lunacy would either dissonant or eerily harmonious.
And as far as Chibi-Usa goes, we have SuperS. Nuff said.
probably would be the best choice but 
would certainly be more funny.
though because she and 
are so different, it would be cool to see them team up.
