Every single fighter consistently can out perform an assassin at this point in the meta with dive potential, mobility, burst damage, and something assassins don't have: durability. No matter what anyone says I still think that the AD itemization is horribly unbalanced in suiting those fighter champions.
Far too many items that are overloaded with defensive stats and utility while sacrificing no damage at all. Notice something? Some of the worst examples of this are currently perma banned around the clock or are at the top of the meta: Vi, Hecarim, Olaf, Darius, Yasuo, and Lee sin just to name a few.
If fighters can display the same amount of burst, mobility, and dive potential as the best assassins while still utilizing far more durability and typically better itemization, then why did most of the assassins get the "Delayed damage" handicap?
At this point assassins are a pretty grey area where their existence and role is almost pointless. I imagine leblanc and zed will continue to be great as they were always the best assassins in the game due to their mobility but I'm not sure about the rest.
My opinion is that defensive/bruiser items are still too easily accessible to AD champions, giving them plenty of offensive stats while keeping them extremely tanky.