Ahhh...this is a bit Campaign Two centered and I'm much more of a Campaign One guy but let's see:
Favorite arc setting: Hot Take, but I'm going to go with either the Underdark or The City of Brass. Underdark because you always remember your first and I really enjoyed how much it just felt like the land/setting was a part of the game as opposed to just where the story was taking place. City of Brass because it showed just how much Matt has his world built and how great he is at improvising. The party decided to go literally out of nowhere and last minute and it felt like he didnt miss a beat.
Favorite Caleb Moment: I cant think of any one moment, but anytime Caleb let's his humor show. One parallel I think you can draw from Vax to Caleb is Liam has presented a character who seems very dower on the surface but underneath, in moments of comfort or vulnerability, can be quite funny. For example, in Zadash when he humored the party and the npc in looking for smut and then bought fresh bread to use as a hand warmer.
Favorite use of magic? Keyleth's feeblemind on Raishan or her use of control weather to basically clear up a hurricane on the way to Glintshore.
Favorite Cad moment? See...this is what I mean by being more of a Campaign One person. Because...while I've seen most of Campaign 2 (I think I'm like 5 behind)...and I dont dislike Cad by any means (I actually enjoy him quite a bit) I'm having trouble calling to mind any specific "Cad Moment." His introduction and the Dead People Tea as well as his general lack of social awareness and experience is super charming and entertaining though so...any OH HIM MEETING PUMAT SOUL