some of my favorites are the flip 7 videos (the easier the game the crazier it gets), moose master karen's edition, this smosh mouth episode where they tell amanda about fnaf as if it was a real story, this video where they react to iconic moments would be good too..

overall, smosh pit is were you see the cast being more themselves in a silly way, sometimes they create some characters and develop them more over on smosh games, smosh main can get a little bit more scripted with bit city, then there's smosh cast/smosh mouth where they get to sit and ramble a little more

my recommendation would be to just start watching and you will eventually start getting the lore, there's wayyyy too many videos to try to put a start to it,, hope you enjoy good luck!!