So far I have these:
- Camp in Devilfire Grove (quick access to Shadowfort, Quarry, and not too far from Aernst Castle)
- Watergod's Altar falls (or Mountain Waycastle), the half-point between Cassardis and Gran Soren
- Heavenspeak Fort (quick access to Soulflayer Canyon, not too far from the Greatwall)
- Bluemoon Tower
For a while I had one in the Witchwood entrance too, gives quick access to Aernst, but it's better to place it at the back entrance to skip the first part. I did that to fight the Metal Golem, since I needed to change Vocation (my magic-based Arisen was helpless). If you don't need to be a melee class though, you can levitate up the back entrance so you don't need a port crystal there, just warp to Cassardis.
I also try to have one in my inventory in case I need to do a quick trip to Gran Soren and back to where I was, though. And I haven't been to the Greatwall yet, but I'd probably put it there instead of Soulflayer since I have no reason to really return there for a while.