I carry a Fisher Space Pen everywhere, but I switched its cartridge for Pilot Hi-Tec-C refills. It takes a little fiddling to get the refill in there, but once it's in it works great!

I commented this elsewhere, but I figured it deserves it's own top level comment.

You can take an affordable stainless steel zebra f701 and swap out the ink with a fisher space pen refill for a solid, utilitarian pen for less than 15 bucks.

i use Mitsubishi roller balls, real nice and inky, great colours. I'm a lefty so can't really use fountain types.

Back when I constantly needed a pen I used to use a Parker espirit telescopic fountain pen, since it was the best small fountain pen that cost little enough that I wouldn't mind loosing, but it was still small enough and good enough for my tastes. More recently since I stopped needing an actual pen with any sort of frequency it just kept getting dry and I just decided to stop carrying one.

This days the only thing that I physically write with any sort of frequency are ttrpg character sheets and for those I just need a good pencil, so I moved to the wsd magnetic 2.0 mm lead holder. I just love it both as a great mechanical pencil and a fidget toy

Not the pen I was thinking of

TUL pens. Super durable as I click my pens from anxiety and I havent had one break on me. Write very smoothly too

Those Zebras that are, like, 1/8 of the volume of what the median pen in this thread is gonna be. I'm not here to pinch hoses 5x their cartridge diameter, half of their length hanging out for nothing, I'm just trying to write. Any non-zero thickness works for that.

Waterman