With the benefit of hindsight, one of my favourite ways to watch the show is paying close attention to Gabrielle's arc. In some ways I think her development as a character goes a lot further than Xena's, and it's a lot starker or less subtle than Xena's. And I think her costume changes are a really good way the show punctuates that progression, it visually sends us the signal for a lot of her more significant pivot points. It's something you don't get for Xena in the same way in terms of the "present" of the show (though you do if you incorporate the show's flashbacks - in essence because she progresses most before the show's time actually begins, whereas Gabby progresses in realtime).

Anyway, all that to say I love them all because they're like little windows into Gabrielle's change. But the most iconic in the sense of recognisable and living in my mind's eye is definitely the s2-3 green and brown combo.