Favourite OVA, hmm...
I really enjoy them all of course.
Although number 9 hit me hard and threw me into angsty feels.
I believe number 8 is my favourite, classic drama of Sonoko being Sonoko, then the hilarious stuff everyone comes up with, and the best part, Shinichi! playing violin too! (even if, as Conan pointed out, he wouldn't realistically show up in that particular manner) It's so cool and romantic with a simple and sweet case. Not to mention, the great karate detective Ran, priceless.
Numbers 1 and 10 are tied for a very close second because I adore everything about KID-sama.