Thanks, Colm. Perhaps I should explain a bit more...
Four and Romana, of course, work great together. Until Rose Tyler, she's the only one even remotely
capable of being Susan Foreman's granddaughter. Romana's also brilliant, top of her class at Time Lord Academy and eventually evolving into E-Space's own Doctor.
Ace brings her Dalek bat of killing and nitro-9.
She's also smarter than she seems.
Jack's immortal, has some nice 'toys' and is pretty badass.
Leela... well, look above.
Also, she kicketh much ass. The Brigadier's military and damned good. He also punched the Master out.
And Sarah also works well with Four. And as someone else told me on another forum, her maxed-out luck stat will enable her to find the solution and get it back to the Doctor.