1. Wrath
2. Legion
3. MoP
4. BC
5. Cata
6. WoD
For me, the quality of the expansion is really about whether or not I want to log in each day and what there is to do when I do. I was totally no-lifeing Wrath, raided probably close to 40 hours per week between multiple toons, had a great guild where people were always down to do things, etc. Legion I have a reason to log in every day, and I love the M+ system, but unfortunately my guild has become pretty shitty and I don't like pugging so I don't get to do those things as much as I'd like. MoP was one of those expansions where I don't feel like I appreciated it enough when it happened but do in hindsight, even with the really cheesy premise. BC wasn't bad, and Kara was an amazing raid but it doesn't rank highly for me. Same with Vanilla. Cata I didn't play a lot after burning myself out in Wrath, but I don't really have much in the way of specific complaints. WoD there was really no point to log on for more than five minutes outside of raid night which is certainly not what I'm looking for in a game I pay a monthly fee for.