This document is old and only partially contains current information. I'll update it.
Generally, I prefer larger refills to smaller ones and those who fit in more than one brand's pens, so mostly I use the ISO 12757 G2 style ones (Parker, Rotring), Ballograf and Fisher with adapters.
I think ballpoints generally don't write very black when they contain black ink, so I usually only buy them in blue and green nowadays.
The wider the tip, of course the shorter the expected write length of a refill. For example, Cross says that a medium will write 20% longer than a bold and a fine 70% longer than a medium.
These are the same size as Parker's, but not same mechanism, so some pens can use either style. For example, in a Parker pen they fit, but can't be retracted because they lack the slots.
Write well, are certificated archival in blue and black and it seems to me the green has almost the same archival properties.
I'd like the green better if it was a bit darker green.
I like the slightly deeper blue compared to Ballograf, and they come in bold which I think usually write smoother than medium.
The black is also good.
Write well and quite smoothly in bold, also archival.
I've become a fan of the BPS-GP XB 1.6 mm pens, or rather the extra broad refills in them. Very smooth writing in blue or black, with good colours. Can be fitted in other pens. My estimate of the write length is 900 m. Some tendency for clotting when carried tip down.
With the adapter they come with, these will fit in pens designed for Parker-style refills, but since they're narrower, the spring won't engage quite the same way in other pens which may be adapted to take ISO 12757 G2-style refills.
I especially like how the bold ones write and the blue is a quite deep blue.
Some people are noting they clot a bit much. My analysis of this is isn't really the ink in itself, but lint from the pocket. This is because the different consistency of this ink makes the exposed ball almost totally covered with ink most of the time. Regular ballpoint inks flow away from the ball.
They do have properties which sometimes makes them the best answer, but most of the time, on ordinary paper and at your desk, they're not much better than regular ballpoint refills and on some glossy magazine surfaces they may smear more easily. (Seems to me the black is worse than the blue in this regard.)
I don't like anything except the colour of the green (I've found other brands' refills also differ between colours, with green sometimes a less smooth writer than blue or black).
I've so far only found one refill which is really good and fits in my pens: Caran d'Ache's. It's their blue I use.
Gel inks have their advantages like generally smooth writing, and disadvantages like cost and the relatively short writeout of the refills. It seem like Parker supports the usual claim they write 1/8:th of ballpoints for similarly sized refills.
They can be pretty permanent and water resistant compared to rollerballs and they certainly write smoother and can lay down a better line than ballpoints, so there are applications for them besides metallics and pastels.
Note that while they may appear to be similar, they don't necessarily have the same properties.
These were the first gel ink refills I encountered and I liked writing with them from the start. It's the black I use.
I don't much like the fine (0.5 mm) refills, but the 0.7 mm and 1.0 mm ones write quite well.
Write length is, according to Pilot, 900 m for the 0.5 mm and 700 m for the 0.7 mm. This compares pretty well with the ballpoint refills which go in the similar pens (like Dr. Grip), but note that these are quite tiny ballpoint refills.
The blue I like best is Zebra's JF-Refill. (Too bad they're not easy to get around here.) Compared to Pilot it's a deeper blue shade which I like better and it's also much more permanent.
A black refill which is slightly better than Pilot's is Niji, as it writes more smoothly. (Not easy to find around here either.)
I also use the 0.7 mm Parker Gel refills, usually black. To a large extent because they fit in ballpoint pens (but I also use them in Pilot G2/G2ex pens), write smoothly and lays down a good black line.
Unfortunately, they're far from cheap (cost the same as ballpoint refills, but write 1/8th).
Writing instruments - >Favourite ballpoint, gel and rollerball ink refills<