Although Lavender Lake doesn't smell too great, in my opinion (I love natural flower scents but she comes off as very plastic-y and artificial) she is a close contender for Best Pony for me, because her blue-and-lavender combo are my two favorite colors. And her cutie mark is very pretty.
But I had to vote for Thistle Whistle! 1.) great name! 2.) so few G3's got such vibrant, neon colors! 3.) she got to be in the animated specials twice and actually had a cute personality that makes her very memorable for the viewers