Dashie is my favorite and has been since the beginning. I remember liking her initially because she was spunky and brash, and was willing to voice her opinion no matter the cause. Its kinda something I'm not, so that might have had something to do with it.
But I think one of the reasons I like her so much is actually her flaws. She can be very arrogant and selfish at times, and definitely has moments where she'll act and speak instinctively with not so successful results. What I also find interesting is that it seems that she knows she has some flaws, but only to an extent. For example, she does realize she can be very competitive, especially with Applejack, and even does admit this from what I can remember. However, she fails to realize her other flaws as of currently, and this imperfection seems to be the one of the more evident of the mane 6.
I also like to think that she has one of the more complex personalities. We know how she acts on the outside, but really don't know much about how she really thinks on the inside in certain situations. She started to open up some during season 2, and even more so during season 3, but she still seems to be hiding an awful lot otherwise. It fascinates me and makes me wonder if there are untold reasons for why she is how she is.
Overall though I find her to be extremely complex yet not so at the same time, and I love her overall demeanor, as well as her appearance. Its almost if the rainbows are symbolism for her complexity.