I like Giyu, but Shinobu is far more unique as a character.
If Dante from Devil May Cry is the epitome a masculine badass-ery, then Shinobu is the epitome of feminine badass-ery, not a character-like Bayonette. Physically, she's notably smaller and weaker than average, but makes up for it with graceful skill and, evident through her poison and medical developments, knowledge of the yang or "negative" (demonic) parts of the world. Personality-wise, she's more of the stereotypical woman, although, in a historically broad sense: Shinobu's all smiles and has a lot surface-level niceties, but underneath that, there's a torrent of emotion, anger and resentment, no less. This is handled wonderfully in the series through her method of killing and initial cold-blooded demeanor regarding demons and Giyu when he prevented her from killing one.
While she would be far from notable outside the series, that alone would have landed her among my favorite characters in the series, but that's not all there is to Shinobu's character. The amazing contrast to that quiet anger, aloofness, and false exterior is a legitimate dedication and care about those in need. In the show, this aspect of her personality is beautifully interwoven with her cold-bloodedness.
Two, especially feminine opposing halves with a badass graceful fighting ability. She's by far my favorite character in the series and one of my all-time favorites.
Tldr: If you don't like Shinobu you just might be gay.
