I don't remember my first fight with an Ancient, but I clearly remember Elder Dragons. They showed me which way things were going to. As a result, I focused on elemental resistance enchantments.
When Ancients eventually showed up, I was definitely prepared, otherwise I would have quite vivid memories of my first encounter with them.
By the time Dawnguard came out and brought Revered/Legendary Dragons along, I was already at level 81 so, they weren't that much of a trouble, even on master difficulty. The game started seeming a bit dull and a fresh start was in order.
So, I made another character. This time, I would make a pure mage. At some point, I've had him try to make a name for himself as a warrior,
without a single point in any armor/weapon skill and just 3 points in smithing (Steel, Arcane & Dwarven). On master difficulty, Revered Dragons would kill that fellow with ease. This, until I started crafting and using potions, other than your usual
"Smithing Phial of Awesomeness" 
I ended up having a lot more fun with this character, than with any other
