OOOoooh, interesting question! With Temptations, there are some obvious ones (If Father Basil didn't come to Hell, there wouldn't be a story, etc) but beyond that, I would say Dandelion would change the most.
If Basil didn't meet Dandelion, he would have easily been eaten by the She-Wolf or captured by a demon or some other gristly fate. There is a slew of AUs potential of which demon Father Basil ends up meeting and hooking up with first. If he had met, say, Hollyhock first and fallen under the demon's influence, the story would've been completely different.
Meanwhile, in Mad Dog, if Malady didn't exist, Lapis probably would've just kept living his con artist life until eventually dying or his past comes back to haunt him in a particularly bad but unsurprising way. BUT if Lapis didn't exist, Malady would've remained under his master's control, never having a chance at freedom, and eventually helping his masters plans that has Bad effects on the rest of the world. It's interesting how that would work out.