When played perfectly, a Janna poses the biggest threat with potential positioning abuse. Good news is that there are very few who are good at that. That makes her a long-cooldown CC (even with maxed out CDR), significantly less effective except as peel. If Yasou is not banned, Janna is a good ban too, because she can trigger his Ultimate pretty effectively, kicking him into that snowball potential.
In my personal opinion, the Vayne will likely be the biggest threat of the 3. Since an average-to-good player should know that she's a later-game champ and will usually stall out a lane unless you have bad positioning. Stalled lanes are just difficult to play around. She scales better with items, which means longer games makes her more of a threat in junction with her stealth, giving her a defensive facet that the others don't have.
Caitlyn is just knowing how her kit works and how to avoid letting her abuse it. She's really squishy when you force her out of position.