It could be said that filling systems don’t matter at all. If the nib is the right size and shape, if the grip is comfortable, if the ink flow is good, what does it matter where the ink comes from? They all work, after all.
Ink-in-the-barrel systems like eyedroppers, piston and plunger fillers have the benefit of holding a lot of ink and not needing filled very often. But is that always a benefit? Those who like to try lots of inks might not think so. Each of these types of pen has the possibility of dropping a blot of ink now and then.
Sac fillers come in for a lot of stick. Some of that, doubtless, isn’t really down to the filling system, but to their age. There aren’t many modern button or lever fillers around. With these systems you’re always at the mercy of the repairer or your own skills. Sacs can be sensitive to some inks too but to be fair, anything is likely to be sensitive to some modern inks!
The cartridge/converter filler is held in considerable contempt by some, especially those in the vintage camp. Such a pen doesn’t really have a filling system – it’s just a shell awaiting a receptacle of ink. They are the modern equivalent of the safety razor of yore that would only accept one kind of razor blade. Not many converters work well. If you try to fill the pen with the converter in place you usually get just a few millimetres of ink! If you fill the converter, then insert it into the pen you have the king of all hard starters. There are ways around this problem but they’re finicky. At one time c/c pens were cheap because the makers knew they would make their money on the ink and to ensure they did, made their cartridges the only ones that would fit the pen. Many still do today but the pens are no longer cheap. Some – not all that many – take the International cartridge. Others prefer to make money by ensuring that as well as a cupboard full of chargers we have a drawer full of cartridges.
So it’s all bad with the c/c fillers, then? Not entirely. I’m writing this with one, my Waterman Carene with an International cartridge. C/c pens are popular with people who like lots of inks because they have the benefit of not holding much ink. Whether filled by cartridge or converter, they are probably the easiest pens to flush thoroughly.
There’s no objective ‘best’ filling system. It’s down to what you like and how you use your pen. Personally, partly because I’m not really an ink-head, I admire some filling systems over others; the Onoto plunger system and the ingenious Ford Patent filler outshine everything else for me. Even among lever fillers, the swing bar system used by Waterman, Conway Stewart and some others strikes me as a far better way of operating a lever filler than the less efficient J-bar. I could go on, but you get the idea. Some pens just met the engineering challenge of self-filling better than others – and some pens just met the challenge of making squillions for their company better than others.