Finding space isn't really a problem, finding a house where you can give up half your slots for a house is if you also want to decorate it. I had my original guild house in Earthtear Cavern using murkmire stone platforms to build a large area at the far end of the house (in the water). I then moved to the Ayleid Ruins house and built on the water there, with the pre-made structures in the house carrying most of the decor. I've done Stillwater's for the holiday and they fit pretty snugly on the frozen lake without needing to use platforms underneath (yay!). I have a few other houses I will move to next and they are all pretty decent for constructing platforms. No need to use Coldharbour unless you are on a budget - which is not a great spot to start as you've got an expensive project ahead of you

Moonsugar Meadow is another good one besides Coldharbour, for guild houses, and the atmosphere is nicer, imo. But the difference is that with Coldharbour you can set stations more or less directly on the surface, but in Moonsugar, you'd need to put some platforms first because the ground isn't flat.

mine use the volcanic island (i forget its name)
i also see poeple use psijic house due to its size

Elinhir Private Arena, hands down.

Yes, it's only available once or twice per year, and it's not the prettiest for crafting stations (unless you like lava).

... But there's nothing like watching your guildmates smack each other to death in the arena while dodging blade traps and fireballs.