People who drop themselves into the abyss of otaku media tend to have their sense for things dull over time. Action scenes don't excite like they used to, rom-coms aren't as funny, ecchi isn't as hot, and only the densest stories succeed in scratching the itch. This is what I (and some others) call becoming a 'jaded veteran otaku'. Jaded veteran otakus often become 'genre specialists' (meaning they only play one type of game, watch one type of anime, and/or refuse to play specific types), and only the most hypocritical have less than ten favorites in their medium.
However, how exactly do you tell that a VN (in this case) is one of your favorites? There are a number of signs.
1. Does watching the opening get you excited no matter how many times you've seen it?
2. Does a great phrase that shattered your personal gate of tears do so again on a second playthrough?
3. Can it still make you giggle?
4. Can it still delight you with new discoveries or even only confirming the ones you made the last time you played?
5. Does defending it to everyone around you fill you with passion and do you have an intense desire to chain every other VN fan in site to a chair in front of a computer to play/read it?
6. Do you find yourself wistfully wondering if you'll ever find anything else like it again?
7. Do you feel like a junkie coming down off a great high after you finish it?
These are just some of the signs of a VN happening to be your favorite... and one of the strange side-effects of being a long-time addict of otaku media is that your favorites become ever more distinct to you as time passes. I have literally forgotten the contents of more VNs than most people have played, but the ones I love remain in my memory with surprising detail, even after years without playing them. Not all of my favorites are kamige (in fact, surprisingly few of them are), but to me they are irreplaceable friends similar to the books I can never bring myself to get rid of.