What's the best refill for a Big i Design TI pen?

I purchased a titanium pen from Big i Design when it was on Kickstarter and one of the main selling points is that it supports refills from many of the high end pen brands. There are too many options, however, and I was wondering if anyone had picked out a refill that they specifically like using with the Titanium Pen. I’m looking for something that writes smoothly and doesn’t require too much effort to put ink on the page.

Thanks.