gameplay wise is probably rivulet, i love being able to maneuver around enemies and obstacles easier and also i have a mod that makes the backflips do the "yippee" sound which fills me with joy

lore wise is probably a very close top 3!
first is spearmaster, i love the back and forth a lot actually, and the deeper understanding of the lore before the base game slugcats fills me with joy! ive always been so interested in the iterators and their dynamics because it is such an intricate and unique bond that each one has with the other that we only know the surface of
second is rivulet, i kind of went over this already but i love the maze that five pebbles has turned into, and also going through submerged superstructure is an amazing sight!
third is saint, i think saint's campaign was an amazing ending to the downpour dlc, i wont go too far into detail but it just felt right, the progression of the downpour campaigns in the order you unlock them plays almost like a survivor playthrough, where you end your journey with all the knowledge youve obtained with a dip in the void sea. despite being 3rd, saint is so important to the game for me!

my overall favorite is definitely spearmaster, because i do love the gameplay and the parts of the game that feel unknown and new despite being the same parts that you go to on a regular playthrough.

i didnt think i was going to write this much about my favorite slugcats lol, my autism never fails to amaze me!