My top 3 in order of worst to better

Ilos: Find the Conduit, Silver Coast Casino: Infiltration and Horizon.

In ME1 I'm fine with the main missions but the sidequests that take place in like two copypasted buildings get boring fast.

In ME2 it would probably be the Collector Ship/Collector Base. Don't get me wrong, I think the suicide mission is a great concept, but I just find the Collectors to be the most boring enemies to fight, and the environments of these levels are also dull to me. Plus Harbinger can't shut up (he became a meme for a reason) and the Human-Reaper is just underwhelming as the final boss.

In ME3 it would probably be Priority: Earth mostly cause it's just more of the same enemies as before with no unique mechanics or bosses. Maybe in some alternate universe where the game's development wasn't rushed there exists a better version of it but as it is, it's more tedious than anything.

ME1 side mission were a pain. I always just played the side missions that had impact in ME2.

For me, it's any bits requiring that pitiful Hammerhead vehicle in ME2. Overlord was a fun and macabre DLC, but the parts requiring that mosquito-with-aspirations was downright molar-grinding. I won't even start on navigating those various planets searching for those idiotic scientists.

For me it the reaper iff misson the story good but all the husk can annoy affer while

For me it's Horizon, Collector Ship and the ME1 and 3 Prologue Missions (Mars excluded)