OK, it's really hard to pick one person (other than OC, as the OP rightfully said).
For example, I've always found Al to be quite an interesting character due to his knowledge, range of interests and wit. Sadly he's keeping somewhat a lower profile nowadays, but is fortunately still around unlike Hash, who's another great mind with equally great sense of humor.
Virginia is definitively among the most interesting minds I've encountered, both on- and offline, having a pleasure of discussing a wide range of topics with her and he grasps on things can blow you out of the water.
Mechaman, Dallasfan (both RIP) and Lumine are the people I had most "creative" fun with, working on games and stories. Never had enough of them.
There are more, obviously, but by mentioning all I'd undermine the purpose of the question even more, given I've already names 6
)