There are quite a few different research tasks in Legends Arceus. From the standard 'Number Caught' or 'Number Defeated with <X-type> moves', to 'Times you've given it food' or 'Number you've caught while sleeping'.
Which research task(s) did you like doing the most and which did you like doing the least?
For me, 'Number you've seen leap out of trees' is one the most annoying/tedious ones when it's on rare Pokemon such as Cherubi. On common Pokemon like Burmy I don't mind it at all.
Tied with it are all the 'Times you've seen it use <insert move name here>' objectives. On their own not necessarily that bad, but pretty much every mon has at least one of them, and as I'm going for a Perfect Dex it's just so dang tedious to do all of these T^T
I quite liked the 'Number you've caught while they were sleeping' and 'Times you've given it food', as they were less common and gave some nice interactions with the wild mons outside of combat. 'Times you've stunned it by using items' was also usually fun to do =D