I've mostly been a raider/PVE-player since the beginning so rating the expansions mostly on that:
1. TBC - Before someone else says it, I admit nostalgia plays a big part here. TBC was my entry into WoW and I had tons of fun doing Kara/ZA with my IRL class mates at the start, and then I had a blast with the raiding guild I joined where I became a raider and eventually classleader, clearing all the raids and having a blast in BT and Sunwell. Overall I really liked all the raids and didn't really dislike any of them, even enjoyed doing Maggy and Gruul mainly because we never really did it, so those rare times we did, it was something different. Isle of Quel'Danas also brought some fun. Great dungeons and great raids.
2. Legion - I know a lot of people disagree but I love the RNG system right now and I am 100% fine with legendaries and TF/WF being the way it is. It keeps me engaged and I want to keep running content, something that was a problem in the past. I think back especially to WotLK and Cata where you didn't have any reason to do content if you already had the gear, and I like the fact that upgrades can come from basically anywhere now. Overall I thought EN was a decent entry raid, ToV was a step up and Nighthold is amazing so if they can keep building on this, we might have a winner in Legion. Dungeons have mostly been really great IMO, even though I get why people hate certain dungeons, I don't really mind doing HoV or BRH for example. Great dungeons, great raids.
3. MoP - I thought MoP was off to a somewhat slow start with fairly boring raids at the start, but Throne of Thunder was one of the best raids in the game and Siege of Orgrimmar is probably the best final raid of an expansion since Sunwell. I had a blast in MoP and for the first time since TBC I was really enjoying the game again, with a lot of ups and downs in the previous two expansions. Looking back at it, I think some of the dungeons have aged slightly poorly, so I would say it's a mixed bag of great ones and slow ones, same with the raids where I didn't really love the first ones, but ToT and onwards was really great. So mostly very positive.
4. Cataclysm - At the time I didn't really like Cataclysm. A lot of focus on revamping old content and I didn't like the fact that the new zones were spread out because you didn't get that same "exploring a new world"-feeling that the other expansions brought. But like a fine wine, I think Cataclysm is easier to appreciate in hindsight. With the characters I've leveled since, I truly appreciate what they did to the old zones. I enjoyed the harder dungeons and some of them were pretty fun, my biggest gripe is really with the raids. I missed the first tier, but having seen videos and done the bosses after the fact, they seemed like pretty fun raids that I was just unlucky to miss out on. Bastion of Twilight and the Blackrock one especially. I'm not on the hype train when it comes to Firelands, to me it was a really ugly raid with some fun bosses, but the idea of a fiery landscape was just so underwhelming compared to a big citadel or some other epic structure. The fights themselves were alright and not bad. Dragon Soul, I guess the less said the better. Still, a solid expansion that was easier to appreciate after the fact than during.
5. WoD - Putting this over WotLK is gonna be controversial (maybe more controversial than putting WotLK last) but hear me out for a second. I agree with the negative points people have about the expansion. Standing in an empty garrison sending missions, few things to do, tanaan being boring and so on. As someone who was mainly a raider, the quality of good raids is something I feel like WoD delivered. Highmaul had a really cozy atmosphere and I had loads of fun doing it, the Foundry raid was my favorite of the expansion and I would put it on that pinnacle of raids like Ulduar and Throne of Thunder of just being really great and defining the expansion. HFC was a solid exit raid. Not the best, not the worst, but enjoyable. I enjoyed most of the dungeons as well.
6. WotLK - WotLK being last is controversial but it was very up and down for me. For starters, Naxxramas was the most underwhelming entry raid. I remember my guild going in hyped, cleared the place during the first night and the feeling of "now what?". The first part of the expansion was too easy. People pugging content that you did in a guild and it felt kinda pointless. By the time they introduced hardmodes/heroic, half the expansion had already passed. I agree with every positive thing people have to say about Ulduar. Amazing place, one of the best raids ever, but for me also the only shining beacon in an expansion of raids I did not enjoy. ICC was OK, but I would put it in the category of a Hellfire Citadel or Siege of Orgrimmar rather than I would put it up there with Karazhan or Ulduar. As far as dungeons go, I think most of the dungeons were alright, but I didn't really like any of them that much. Whenever I go through the expansions when leveling a new char, there are always those dungeons I am looking forward to. Like TBC has several of those, but whenever I come to the WotLK levels, the dungeons are mostly alright, I just didn't click with any of them that much. Otherwise I agree with people that Northrend as a continent was pretty neat, atmosphere was cool, but as a raider, a lot of content was too easy but more importantly, I just didn't like much of it. Ulduar was amazing but apart from that the raids were really boring IMO. ICC = OK, Naxx = terrible shadow of its former self, TotC = worst raid in WoW and so on. Dungeons were mostly decent, but I still have to put WotLK under Cata and WoD because despite the incredible level of Ulduar, I feel the consistency wasn't quite there. For me at least.