LEAST favorite group to face

This might depend on who you are facing. It will come as a surprise to some people but there are a lot of guilds that are not fully heroic and therefore don't have Traya.

If you are facing a non heroic sith guild the Geonosians might be the tougher choice. More casual players in non heroic guilds are not as up on how to handle newer comps like Geonosians.

Against a guild that has Traya then Nightsisters are a solid choice, very few people like going against them.

Third would FO - assuming you mean KRU lead

Phoenix would be fourth, once you reach a certain point Phoenix simply are not something to be worried about going against.

I assume we're talking about a back territory after we've already used our meta squads on the front row? Definitely Geos. NS are a very close second though.

Correct. Although a genosian or nightsisters front might be interesting.

Nightsisters by far. Geos is easily beatable with traya and sion + friends. If it doesn't work get equal or better gear/relic. Easy win.