I've seen all the OVA's so far and my favourite one's are 2 and 10. To me OVA 2 is nice because of the relation everyone kind of shares between each other (with their typical personalities, jokes and ways of being) and even moving because of the moment both ran and conan remember. It's simple but I love it. And then I would say OVA 10 is so funny, it makes you laugh and enjoy the whole vid (Kid's just hilarious). And about OVA 9, personally, I liked it.. yet it made me feel so uncomfortable, I couldn't deal with the idea of that becoming true and happening for real one day. But I believe too that the moment when shinichi tries to tell the truth desperately to ran is just so touching.