And Pocahontas, but for different reasons. Pocahontas is the best DP when it comes to actually sneaking around, but another part of being a spy is being able to manipulate your enemies, which I think Jasmine is the best at. (seen with Jafar)

She's quick-witted, agile, and strong (both mentally and pysically). Just think of the opening shark scene and how she saved Erics's life (pulling him up to the surface and swimming him all the way to land). I think Ariel would be a great spy :)
I also agree with ppgbelle4 and I think Mulan would be a good spy as well :)

I agree about Ariel too, especially when it comes to being agile and strong. Being a spy has to require a lot of stamina, and she HAS to have great stamina from swimming every day in the sea.

She managed to keep her gender a secret for months, and was able to catch up with work that is normally suited for men- even becoming better than half of her fellow soldiers. She developed a talent for quick-thinking as well.

For Jasmine, I think yes, in a way she would be great when it comes to maniuplation. Although when she was trying to hide who she was, she easily let her hood slip and showed her hair-piece (I'm assuming it's a crown), and it wasn't long before she did tell the guards who she was- it was called for, but it sort of shows how easy she would allow her identity to be known given certain situations.

Mulan and Pocahontas are my top choices.

She's kinda like a chinese ninja. She's trained to do this kind of thing, so she'd be familiar with it. But I think she'd want Shang to go with her also, lol. She can probably do the best job sneaking around and investigating, she's already pretty good at it.