Usually in JRPGs status effect spells don't work on bosses to keep some challenge in the battle. It can make a fight disappointing when you just paralyze the boss over and over. It's unfortunate since it removes potentially useful tools the player has.
A lot of Final Fantasy games have the problem where normal battles are too easy for status effects and the bosses usually are immune to them. Harder JRPGs like the Shin Megami Tensei series and the Etrian Odyssey series make status effects viable but account for it with increased difficulty and emphasis on team composition. I'd also say Pokemon has really powerful status effects that really work due to the battles being one on one.
The funny part of the "vile ingredient" is that its annoying when these spells don't work despite using resources on them when a normal attack could have made the battle come to a close sooner, making one wonder what is the point. I think it would be fun to mess with the mp costs of these spells to fool people into trying them or have a boss give clues to being susceptible to an ailment when its just a big lie.
To answer the question of my personal favorite status effects: Slow in Final Fantasy 4 DS, works on every boss and is very useful in a game with the active time battle system. And poison in Etrian Odyssey 4 due to dealing a lot of damage per turn and having character classes that do extra damage to ailing enemies, allowing you to make a party that focuses on causing various ailments and then capitalizing when they land.