The dungeons I'd rank from easiest to hardest, back when I was doing them:

1. Waycrest Manor - off sanguine and quaking weeks this has to be the easiest of the bunch. The boss mechanics are all easy to do multiboxing and the trash isn't obscene. Could be tougher on sanguine and quaking weeks as much of the fighting happens in tight corridors.
2. Temple of Sethraliss - this one is also very easy for me. For much the same reasons. The boss mechanics are manageable and the trash not too bad. It is longer than WM though.
3. Freehold - Some of the trash here starts to get annoying. Lots of grips and knockbacks and knockdowns to be aware of. Bosses are easy.
4. Motherlode - Lots of annoying trash you need to contend with and somewhat long but the bosses are all manageable.
5. Tol Dagor - manageable trash. Not terrible. Some bosses are challenging. Last boss can be if you are learning the encounter.

Past this point it becomes a lot more dicey.

6. Atal'Dazar - super annoying trash and boss mechanics go up a notch or two. Vol'kaal will ruin turret comps (DPS that can't DPS and move) and Yazma will make your life hell on movement as well.
7. Shrine of the Storm - super annoying trash and difficult boss mechanics on the 2nd half of the dungeon.
8. Siege of Boralus - last boss fight is just odd to me and incredibly difficult.
9. Underrot - reasonable trash, impossible boss mechanics. Especially 2nd boss.
10. King's Rest - just a garbage dungeon through and through. Crazy trash and bosses are tough.

Mechagon is somewhere between 7 and 9 for me.