Restricting yourself to a twisty, I would concur with the Parker Gel Refill.
However, I like the Cross Ion Gel Refills. With these, you can get somewhat of an idea of how much ink is left in the refill because they are made of translucent plastic. The Parker refills are metal. [In theory, the Parkers might be less likely to leak.]
If you're accustom to using ballpoints, rollerballs and gel ink cartridges have a fraction of the write-out of ballpoints. That is why I would prefer the Ion refills. The quality is identical in my opinion.
Cross also makes gel refills for their rollerball [selectip] pens, but they are metal like the Parker refills.
[For the most part, I am a Fisher ballpoint person, so my opinion probably isn't worth that much anyway.]