My problem is really narrowing it down - so many passives are useful in this game. There are a handful I shake my head at, but for the most part they add a flavour or enhance gameplay for champions.
There are some that are not quite as useful, overall - Ashe's passive is great for ambushes, but overall her crit chance lategame is going to be high enough that her passive just falls off save for a guaranteed one every first hit in a fight.
Taric's is poor in the 0 CS meta, but if one was to build him bruiser it'd be awesome.
Kassadin's sometimes feels a bit lackluster, but then again, I'm a bad Kassadin - I appreciate the reduced magic damage but rarely use the AS component
Leona's adds a lot of free damage early, but by lategame, it's almost negligible
Miss Fortune's is getting buffed here and there, but still feels weak seeing as it simply lets her arrive places, and then won't contribute in a fight
Zilean's is great early, falls off late (obviously)
Nunu's is god-like in the jungle and only slightly useful as a support
Most Orianna players count on the ball to help them deal damage, auto-attacking is probably nto second nature and so this passive, while strong, is probably overlooked
Sion's sometimes feels like it's not helping much
Viktor's needs lategame usefulness, though the choices to your gameplay are nice, even if 95% choose Death