For ballpoints, I recommend Schmidt P900 refills available from a number of pen suppliers like Exotic Blanks. These are very popular with advanced pen turners. I recommend these for an upgrade refill to any kit pen using a Parker style refill. They are available in varied colors and widths. For Rollerballs, I suggest Monteverde refills. Also higher end and available in different colors and widths. Schmidt also makes an excellent rollerball refill - their #5888 or #888. All upgrade refills are between $1.10 to $1.70 per . Replacing the refill is a great way to upgrade your pens.
If you choose to move to fountain pens, Schmidt converters with Jowo or Bock nibs are the way to go. They are the best upgrade for the cheaper ones provided in kit pens.
Kevin Seiler