Not enough discussion here. Rankings are dull.
I'd say the original quartet was best, obviously. But why? Quinn and Wade were terrific youthful counterparts to Rembrandt and Arturo as the older generation. You had daring youths contrasting with the more conservative characters.
Also, the characters were devised in a way that any pairing of the quartet would produce an interesting combination. Pair up Wade and Rembrandt: you have Wade's curiosity and wonder and Rembrandt's astonishment and life experience. Put Arturo and Rembrandt together and you have the academic and the artist. Pair up Quinn and Arturo and you have a son and father exploring the world through science. And so on.
With Maggie replacing Arturo -- it didn't work, but that was mostly because all the characters were being very badly written. But looking at what it could have been -- Maggie could have been a terrific addition.
The key, I think, would have been to write Maggie as being about Rembrandt's age -- a woman in her mid to late forties and with all the cynicism, practicality and life experience of a master spy. I think a more realistically written Maggie would have regarded Quinn as a youth in need of a mother / older sister, to see Rembrandt as a loving but ineffective father, and to see Wade as her protege in adventuring.
The Season 4 team was an odd misfire with Cleavant's character having lost all comedy and Jerry and Charlie no longer acting.
Season 5 had a lot of potential. In many episodes, Rob, Kari, Cleavant and Tembi seem to have amazing chemistry. I don't quite know how the characters could have worked out, as they were scripted as bland non-entities for the most part. But there was definitely a lot of raw material and great talent there.