If you're talking about an organized structure:
* Chaos: This implies a lack of order or structure.
* Disorder: Similar to chaos, but emphasizing the absence of a planned arrangement.
* Randomness: This suggests actions or events happening without a clear pattern.
If you're talking about a method or process:
* Haphazard: This implies a lack of planning or organization.
* Unsystematic: This directly emphasizes the absence of a systematic approach.
* Improvisation: This suggests doing things on the fly without a pre-defined method.
Other possibilities:
* Individuality: This emphasizes the uniqueness and independence of parts, rather than a cohesive whole.
* Spontaneity: This suggests actions taken without pre-planning or structure.
The best opposite for "system" will depend on the specific situation and the meaning you want to convey.