Is There a Doctor on the Dig? by Bat Morda features Janice and Mel travelling to a remote island to get the scrolls back from Janice's nemesis. At the time it came out, this story was the be-all and end-all of fanfic to me. I adored it. Rereading it 10 years later, I can only say that femslash was still pretty new (at least to me) and I was just a teenager to boot. I still think it's a good read, but I could do without the cheesier elements (eg. naming practically every single character after someone on the show or in fandom, naming the Callisto character Cal Leesto).
The sequel
The Search for Amphipolis does nothing for me, but the third installment,
Ubermadness (which, as the title suggests, crosses over into true uber territory), wins me over with its sci-fi elements and the interaction between its protagonists. They're reincarnations of Xena and Gabrielle, but only Gabrielle knows it and has the bard's memories, which puts her in a pretty painful position when her soulmate has no idea about their past lives or their connection to each other. Janice and Mel show up as guides to help the reluctant uber-Xena regain what she's lost.
The Party is technically another part of the series mentioned above, but this PWP stands alone. Janice and Mel go to a costume party. Then come home and have great sex. This, to me, is the model for all PWPs. Excellent.