I will echo the sentiments of Eldergleam Sanctuary and Blackreach. There are many others, but hard to think of them off the top of my head, I'm getting old.
I found that cave by the Thalmor Embassy last night that was mentioned as well, but I haven't gone in yet. (I was on my way to kill Elewen just because I don't like her and I didn't want to get sidetracked.) I will have to go check that one out.
Quick side note since you mentioned it Seager - what do you mean the dragons stop appearing after the last one? I killed Alduin and still run into random dragon spawns. Did I misunderstand what you are saying?