Ariel and Caliban
Ariel and Caliban provide contrast to each other by virtue of how they got and bear their position as servants. Prospero did Ariel a favor, so Ariel's servitude is something of his paying off his debt in the interest of earning the ultimate prize: his freedom. Caliban earned his servitude by trying to rape Miranda. He is not remorseful for this great wrong, and, at the end of the play, the best he can hope for is to be once again in his master's good graces – freedom is beyond the fold for him.
Looking at what each might gain by his service illuminates how each character feels about his servitude. Understandably, Ariel is glad to do service because it will be rewarded, whereas Caliban's servitude is a constant reminder of lost freedom. As long as Prospero is around, Caliban will never have freedom again. Where Ariel is diligently working towards a goal, Caliban is serving his time as punishment.
Gonzalo and Antonio/Sebastian
Gonzalo provides a good contrast to Antonio and Sebastian because he is happy to be in his position and doesn't wish ill on anybody else to gain a better one. Accordingly, Gonzalo is always full of "the silver lining" talk while the other two are so jaundiced by their position and scrabble for power that they can only taunt and plot.