In terms of the battle itself, the easiest to defeat is the Northern Hunter (Crestwood) followed by the Abyssal High Dragon (Western Approach). But both require some side questing in order to access the dragon.
I'd rate Crestwood as the easier/more fun of the side quests. The Abyssal is just a bunch of go-fetch quests, and since the beasts you have to fetch from don't always drop the thing you want, it's aggravating. In Crestwood, if you do it without glitching, you fight through a fort. If you do it with glitching, there's a path around the fort directly to the dam where you drain the lake. Draining the lake wakes the dragon and sets up the battle.
The lowest difficulty dragon that has the easiest access, no side questing required, is the Greater Mistral in Emerald Graves, but that battle is the first one I'd rate as difficult, unless you are suitably geared and leveled up -- excepting the Fereldan Frostback of course, which is presented out of sequence in terms of difficulty. No dragonling adds in the Greater Mistral battle, but it has that nasty one-shot AoE instant death landing attack. You can open the Emerald Graves and ride mounted to the dragon battle area without fighting anything in between -- though, that is not to say such a ride is easy. It took me a few tries to figure out a path that avoids a rift that's in the way, not to mention a few Freeman of the Dales roadblocks. It helps to have both of the dismount buffs from Perks, but you don't need them. If you get dismounted by a Freeman archer, just run like hell away from the encounter, preferably in the direction of the dragon. 
If anyone is interested, I can do a run and take a screenshot of the path on the Emerald Graves map.
The Sandy Howler is actually even easier to access, just a simple mounted ride with a much simpler path through the Hissing Waste, but it is a much more difficult battle, 8th out of the standard 10, where the Mistral is 5th. You can even avoid fighting the Wyverns that guard the entrance, if you jump your horse down at the right place.
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), it gives a nice RP segue into Bull's after-fight scenes. He also has - I believe - one or two small extra lines with that dragon. But I also feel it is the easiest dragon to take down (with Crestwood's being a close second).
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