The refill is the heart of a pen. This is where ink meets paper and will determine how effortlessly your pen glides across the page and what kind of line you draw. All of my pens use standard ink refills that are readily available from office supply stores. The one thing that I noticed while researching refills is that there is a wide range in personal preferences. What works best for me may not be your favorite. I will ship your pen with a top quality refill installed. Try it out and see how you like it. I've mentioned some well-regarded alternatives below if you want to experiment.
For Pens Taking Parker-Style Ballpoint Refills:
For pens that take Parker style refills, I recommend the Schmidt easyFLOW 9000M (also marketed by Private Reserve), which is made in Germany. This is considered by many to be the finest ballpoint refill available, and I agree. That is why I include it in all my pens that take Parker-style refills unless a different refill is requested. Schmidt uses a lower viscosity ink that flows more readily and has re-engineered the cartridge to prevent the ink from spilling or drying out. The result is a refill that writes like a rollerball, but has the convenience of a ballpoint. I carry the easyFLOW 9000 refills in both black and blue ink. Please let me know which you would prefer with your pen. I have also recently tried the Monteverde Soft Roll refills and find them to be very good. If you prefer a thinner point, their extra fine version is excellent. For those who prefer a gel refill, Monteverde makes an excellent Parker-style gel refills in multiple different colors. Visit your local office supply store. The Parker-style refill is the most common refill type for pens so you'll have lots of choices. I stock a limited supply of refills if you have difficulty finding what you want elsewhere.
For Pens Taking Cross-Style Ballpoint Refills:
Cross-style ballpoint refills are thinner than Parker-style, so most of the thinner diameter pens take Cross-style refills. I find less difference among brands in these refills than with the Parker-style. My favorite is still the Private Reserve brand from Germany. These write smoothly with no skipping. The Monteverde Broad Point is also a nice writing refill. I also find the Dyacom-branded ballpoint refills to write really well. These are made in Taiwan using German ink and Swiss tungsten carbide rolling balls. The Fisher Space Pen refill is pressurized and has many good reviews online. Having tried it, I don't think it is worth the premium price it carries unless you routinely need to write upside down or have a space mission coming up. I have a few of these if you'd like to try one. Cross also makes a "Broad Point" refill that writes smoothly and leaves a wider darker line that many like.
For Pens Taking Rollerball Refills:
Schmidt makes two safety ceramic roller refills (Schmidt 5888F and Schmidt 888F) that get high praise. The fine points are often preferred in rollerballs because the high flow rate of the liquid ink leaves a wider, darker line. These rollerball pens will also take Monteverde Fineliner refills. These are fine point felt-tip style refills that come in multiple different colors.