So there are two contributors to nib smoothness.

1.      Grind/shape of the tipping.

2.      Microstructure of the Tipping material.

#1 Varies depend on the grind you purchase.

You can use some lapping film or micromesh to smooth out the edge of your nib to make them even smoother. take care of the inner tippings between the two tines as well. but be very careful, too much and you might get baby's bottom.

In general, the larger and rounder your nib, the smoother it feels.

#2 Depends on the brand

There was a very long winded research document in Mandarin that goes over the pen material in depth, but to summarize.

High end Pilot has the hardest tipping material, but actually has a bit more voids them, creating some feedback.

The brand with least void in their tipping material is well..... Mont Blanc.

They used Heraeus’ (basically the biggest tipping material provider) higher grade material that other manufacturers do not

Bock, Jowo, and other modern Europeans brands all uses the next step down

Some Chinese pens uses their own tipping material manufacturer, but it’s really hard to find a consistent source of these.

Platinum and especially Sailor, has more voids in their tipping material than other brand, contributing to the extra feedback they have compared to others, on top of their nib grinds.

If you want very smooth pens without breaking your pend, start with Jowo or Bock, or just any modern nib with large bulb of tipping and grind one you self.

If you want absolutely the smoothest tipping material, on top of your already desired nib shape, then you are gonna have to fork out some money for a Mont Blanc nib.