Schmidt P8126 refill
This is the refill you can find in the Retro 51 and a bunch of other pens.
I think it is so popular because of the Retro 51, and have kind of become the goto refill for a lot of pens.
It isn’t for everyone though, I like it a lot and it matches my preferences very well. But it isn’t the best for people who prefer something finer or less wet or more colours.
This might not be what you use a lot. I don’t anymore. But I recommend something that use it to non pen addict people all the time. And most of them are really happy with it.