Only quality prompt that actually improves the image are these:

masterpiece, best quality, very aesthetic, absurdres,

The only negative prompt that improve image are these:

lowres, worst quality, low quality, bad anatomy, bad proportions, signature, watermark, patreon, artist name, twitter username, simple background, borders,

All prompts tags that goes with score or source, will NOT work! Those only work on PONY models!

When prompting you should always order it like this, artist tag, 1girl/1boy, description of character, items that are using or holding, expressions, scene, effects, quality prompts:

EXAMPLE:

(redrop:0.7), 1girl, white hair, red eyes, maid uniform, apron, holding broom, middle finger, angry, anger vein, cafe, day, colorful, contrast, masterpiece, best quality, absurdres, very aesthetic,

My recommendation when not knowing what tags to use, is to check danbooru or gelbooru.

https://danbooru.donmai.us/wiki_pages/tag_groups

PS: Keep in mind this is a generic case for most Illustrious models, there might be some models that requires extra quality prompts to get the full of it, in that case always read the instruction written in the description!