Whatever you define as interesting. I think it'd be cool if we all shared some information on the covenants we've worked with (either playing the Magi in them, or SG'ing the stories surrounding them) that we considered the most interesting and/or fun, and maybe briefly discuss which of their features made them the most interesting and, if this pertains to why you found that feature interesting, how that feature supported the telling of stories your troupe wanted to tell. For one, it's just cool to share fun things, but maybe this can also stir up some creative juices if anyone's hit a rut.
I'll start, though I don't consider any of the covenants I've SG'ed for particularly innovative. My players adore mixing high fantasy gameplay with mundane pseudo-politics and seeing what happens, so our covenants all generally fit that theme. Things like towers of flame within a regio, which is itself within a major city (in that example, Paris), or buildings suspended from vines in a faerie wood with sentient supernatural trees near the residence of a noble who is also an ambitious hunter. The latter is my personal choice for most interesting covenant.
I found it so interesting mainly because it so clearly embodied everything the troupe was striving for in a covenant. The willingness to surround themselves with and engage with the fay allowed for that high fantasy vibe to be pervasive in nearly every story that was told, while the nearness of the nobility allowed for careful politicking as they tried to keep the (rather competent) hunters out of the faeries' territory without drawing attention from the Quaesitors. Plus, knowing the fay, it gave me as SG the freedom to try for nearly any story I thought the players would like without it coming off as contrived. Lastly, it interested me because the players experimented with some new things; for example, that was their most creative use of Magical Fortress to date, what with the trees using minor magics to carry and protect the covenant. It was really rather great, though not the most unique idea I've encountered by any means.
The most interesting covenant I used as storyguide was Val Negra... a variant of the Val Negra Covenant written for the old 2nd edition "covenants" edition. I had to flesh out some things, add some things and change some things (because the saga is already 40 years old and the provence tribunal isn´t provence as we knew it, anymore).
Val Negra in my saga is not only a winter covenant, it is a dead covenant. No living soul is here anymore. But you can find:
some very slow turtles that can lead the characters about invisible bridges (whose invisibility is powered by the energy of bound spirits) and regioborders
a daily battle between the old grog-ghosts and faerie attackers (the entanglements of the adventure "fall and rise" in ToME)
a Criamonmagus in the last state of his "path of strife" (with a golden apple, lifeless in his laboratory)
three doubles of a Tytalusmagus. The magus is dead, his doubles still exist. The one in the garden thinks, he is a gardener, the one in the library thinks he is the librarian, the one in the laboratory thinks he is a cool necromancer (but they all are nearly mightless)
the old experiments of the said Tytalus. He let fight spirits of his dead sodales against demons and nature spirits in a well warded area to learn what can be learned by watching the fights... of course he was just another mad scientist, who wanted to bring salvation for humanity but erred fatally.
stories about a grog that escaped the going down covenant (he went away with a magical flute, the reason for the death of the magi - later the characters will meet the flute again in the adventure "a musical murder mystery")
A life raft made of magical stone, powered by the will of the pilot - you only need to sit on the right place. The boat could be launched to a river.
The story was the following: The characters came to look after the remains of the legendary covenant - slow entrance with the help of the turtle guides - searching for informations - avoiding the daily battles - pilfering the old laboratories - "lay to rest the haunting spirits". The last action led to a victory by the entanglements the next day, they had no more enemies. The faerie creatures reduced the covenant to ashes. The characters had a dramatic escape by the life raft.
It was a very cool story in an apocalyptic atmosphere with the characters as the "last boys in the ruins".
One saga I was storyguide for had the covenant in a glacier in Novgorod. The covenant lived in tunnels in the glacier, and there was a regio that was accessible pretty much randomly. The regio was very, very cold and what they didn't know was that it was a left-over piece of Ymir's beard, still chock full of ice-trolls waiting to be thawed out. Nobody was ever dumb enough to dig one out, though, which is a shame.
The covenant was about a century old and had getting promoted in status relied upon making one of the Hearthstones, which magically provided comfort for the people in the glacier - each Hearthstone was a greater device with several minor enchantments, and you got to keep yours unless you left the covenant, so many of them had extra enchantments as well.
I was only involved in Andorra at the start and stayed only a short while, but it's a great covenant.
A lot of the ideas for Val-Negra and Provencal were generated (although not all of them were adopted by any means) as a result of enthusiastic discussions on the Andorra sub-forums.
I played a Saga called A Covenant In Three Parts. The covenant was built around a Regio that opened onto three distant parts of Mythic Europe, a lone tower/water mill in scotland, a venetian townhouse, and a Pilgrim's hostel in the holy land. It started as a compromise, but turned into a great saga. With a different ST running stories out of each location.
