I'm of two minds about Thessia.
The Geth Consensus mission was brilliant, because it introduced us to a lot of geth-quarian lore, which revealed even more of how the quarian military is made up of fascist crazies who'd happily kill their own people for even the silliest of reasons. I was getting hints of that in ME1, ME2 started setting off alarm bells, and ME3 provided a finality to that growing feeling I'd had anyway.
Operation: Earth was probably my least favourite, mostly because I don't think it reflected the war assets enough. One person made a case that if you save both the quarians and the geth, you should have Rannoch fleet reporting in, made up of both species; and you should get to talk to the prime on earth as well. There are some brilliant lines in the data files for speaking to one, so I can only imagine it was an oversight or limited time that left it out.
So Operation: Earth was definitely my least favourite part. I was glad to get to the catalyst (and I know I'm one of the few people who enjoyed the ending). Thessia... Thessia. Hm. See, I love the aesthetics of Thessia, don't get me wrong, there. It's a breathtakingly beautiful planet, sometimes you just have to stop and look around the vistas presented by the skybox. Even in firey ruins it's still a hauntingly beautiful place, and one of the best meshes of fantasy and sci-fi I've seen in a videogame. Those spires...
But the mission? Walk. Shoot. Walk. Shoot. I almost fell asleep. The problem with me is that I'm the reverse of most people. "HUH? Did something interesting happen?" That moment for me won't be when the shooting starts, but when it ends. I'm the same with movies and TV series, I'm much more interested in the day to day life of people in a fantastic world, or the adventures of them, rather than them killing each other. Action is just a boring and monotonous thing, it needs variety to break it up.
Thessia didn't have variety. That was the problem. There was no Thessian lore. If it was a la mode of the consensus, with lots of information sprinkled in, it would have been more tolerable. But there was no lore, no dialogue, no new gameplay mechanics, and it was just boring. If I want that kind of thing I can play multiplayer, and multiplayer has the dynamic of other people (which I enjoy) and being able to play as aliens.
And it resulted in a genuinely dull 'boss battle' that I could have done without. I'm really not a fan of them and they just feel like they're filler at this point. Videogame content should be about variety, but all boss battles comprise of is memorising one or two patterns and then repeating them over and over and over. This is why in old arcade shmups, the stages were always more interesting than the bosses (in some cases the stages were the bosses, and cleverly done, and in that way they were enjoyable). See R-Type. But I digress.
So Thessia was beautiful but Thessia was also boring. It wasn't the worst, that's still Earth for me. But it was definitely second-worst, and outshone by the rest of the game.