I couldn't really pick one from the selection that was given, so I added a few to the Poll ... and here's why.Most of the time, my fulcrum is in between the two joints (knuckles). I never slide back to where it's on the second joint because it closes the hand up too much and creates a natural tension in the thumb muscle. It makes my hand find it's locked down rather than relaxed. I want it to feel like an extension of my hand, not like I'm getting ready to club something to death.

Here's how I find the best fulcrum placement:
I let my hand hang down at my side, totally limp ... like I was dead or something. If I look down, my thumb is lining up with the flesh part of my index finger ... between the first and second knuckle. While still relaxed, I slide a stick into my totally relaxed hand, then apply just enough pressure with the thumb to keep the stick from sliding out of my hand. I lift my arm up and BOOM ... that's how I hold the stick.
For me, the more natural and relaxed ... the better.
Each person is made differently. We all have different shaped hands, different length of fingers, etc. I would encourage you to try this little experiment and see what happens; you may be surprised just how off your present fulcrum is, compared to the natural placement of your thumb and index finger with a totally relaxed hand.
This is my "standard" fulcrum; my starting place when I first pick-up a pair of sticks. From there, I may roll the stick out so that it's on the first knuckle (for certain things), like when I play a really soft buzz roll. I can drop the rest of my fingers and get the least amount of "me" on the stick ... which seems to work for me when I play very soft.
For really hard playing back beats, my fulcrum slides back between the second knuckle of the middle finger. Most of the time it's almost a shared fulcrum between my "standard" fulcrum and this fulcrum. Doing this allows the stick to feel like it's an extension of my body, coming from the center portion of the hand ... really nice for stronger and/or louder playing.
I never have a fulcrum at the second knuckle of the index finger for the reasons I already stated; tension and closing up the hand too much (for me that is). The second knuckle of the middle finger seems to do this better (for me), allowing the hand to stay relaxed and open, let having the strength and control to play strong/loud.