Uniball's standard gel refills are smoother than most others, and the "bold" version is even smoother.  A larger ball rolls over rough paper just as larger tires are better for rolling over rough terrain.

I still prefer non-gel rollerballs.  A good non-gel roller, such as the Parker IM, writes with no pressure at all.  I am sometimes not even sure that the ball has to roll.  However, Uniball's refill, even requiring a slight push, feels just as smooth.

On top of being a very good gel refill for a low price, this particular model happens to fit most pens designed for the standard Hauser or Schmidt refill even though it isn't advertised as such.  Not only is it difficult to find rollerball refills which aren't dried from storage (or just junk, like Monteverde), but for some reason everyone is always selling "fine" point versions.  Uniball's bold refill isn't an exact replacement if you normally use liquid ink, but it's worth trying.

A few things to note:
1. Not all Uniball refills are the same size, so don't just grab any of them.
2. The front end is different from a standard refill (lacks that little block portion), although it fits all of the pens I have.
3. The metal tip around the ball is a little narrower than most refills, and you may want to wrap a layer or two of clear tape around the tube to keep it from wiggling.

Pens I have which fit this refill:

Waterford/Marquis Arcadia (capped model)
Carbon fiber rollerball from Office Depot (either Yafa or Foray brand, can't remember)
-[bleep]-
A couple generic (but nice) engraved promo pens

EDIT:
I originally reported that I used this in the Cross Century Selectip.  I must have been nuts or lost track when shuffling through my large pile of pens, because it definitely does not fit.
Uniball's standard gel refills are smoother than most others, and the "bold" version is even smoother. A larger ball rolls over rough paper just as larger tires are better for rolling over rough terrain. I still prefer non-gel rollerballs. A good non-gel roller, such as the Parker IM, writes with no pressure at all. I am sometimes not even sure that the ball has to roll. However, Uniball's refill, even requiring a slight push, feels just as smooth. On top of being a very good gel refill for a low price, this particular model happens to fit most pens designed for the standard Hauser or Schmidt refill even though it isn't advertised as such. Not only is it difficult to find rollerball refills which aren't dried from storage (or just junk, like Monteverde), but for some reason everyone is always selling "fine" point versions. Uniball's bold refill isn't an exact replacement if you normally use liquid ink, but it's worth trying. A few things to note:
1. Not all Uniball refills are the same size, so don't just grab any of them. 2. The front end is different from a standard refill (lacks that little block portion), although it fits all of the pens I have. 3. The metal tip around the ball is a little narrower than most refills, and you may want to wrap a layer or two of clear tape around the tube to keep it from wiggling. Pens I have which fit this refill:

Waterford/Marquis Arcadia (capped model)
Carbon fiber rollerball from Office Depot (either Yafa or Foray brand, can't remember)
-[bleep]-
A couple generic (but nice) engraved promo pens

EDIT:
I originally reported that I used this in the Cross Century Selectip. I must have been nuts or lost track when shuffling through my large pile of pens, because it definitely does not fit.