There are many factors involved including upbringing, siblings, culture, regional culture, sexuality, which religion, if any, touched your life, etc. As for me, I'm not bothered by the personal space of one's body. You can bump me, jostle me, stand next to me, and I'm not bothered. However, if you get in my face, then I'm bothered. If you stick a finger in my face, go face to face, shove something in my face, then I'll pull away. Therefore, I feel the most intimate degree has to do with your face.
With oral adventures, one has to take his or her face, and put it on, in, around, or down the other person's most private body part, cock or pussy. That has to be #1. Remember the first time you ever thought about cock sucking or pussy licking? Remember how icky it sounded? #2 has to be intercourse while your kissing. Kissing does ignite sexual passion, but for me, it also helps maintain it. I love wild kissing during intercourse. #3 has to be intercourse itself. Your two most intimate body parts deep within each other. One could argue that this is #1, since this is the way to offspring. #4 would be kissing, #5 touching, and finally conversation.
Where does nudity play into your list? Are you comfortable getting naked in front of others? In my travels I quite often visit hot springs. It's interesting watching the people as they come and go. Some people have no issue taking off their clothing in front of total strangers, with others keep most of their clothing on while sitting in the hot water. Most people are somewhere in the middle.