IMO, the most annoying crafting stations to reach, and the first ones I did at my house, were the 3 sets that are crafted in Imperial City.

Granted, it is a little easier now that you port directly into IC, but when you had to queue for Cyrodiil, hope you had a castle near one of the sewer entrances, go into the sewer, take the ladder up, and then go to the crafting station, all while not getting killed by an enemy... yeah, they were a pain.

Other option would be the sets in the towns in Cyrodiil, since you need to have control of the town to use them, so you don't always have access to them.

My first thought was also Hundings Rage from a "usefulness" point of view, but it is really easy to access. The crafting station in Reaper's March is just north of the wayshrine.

For difficult to access, I'd agree that the PvP sets are the most challenging, though I'm not sure if any are in high demand by PvPers at the moment. There are also the Earth Forge sets, as you must go through a portal in the fighters guild after first doing those quests to access them. Or Morkuldin, which is both DLC and quest locked (though perhaps no longer, see the other thread).

For a mix of the two, the most popular DLC gated set is probably New Moon Acolyte (Southern Elsweyr), though I saw a wave of PvPers asking for Fortified Brass (Clockwork City) not too long ago, and people asking in zone for access to attunable crafting stations from Blackwood.

I guess, take that all as you will, it really depends on how you're planning to use it. If you're crafting sets for new players, maybe stick with Hundings or NMA. If you want ease of access for completing writs, start collecting attunable stations for PvP and quest locked areas. If you plan to open your house for friends and guildmates, perhaps one of the DLC or Chapter locked stations. Hope that helps