A couple of years ago I spent a while looking to try every rollerball fountain pen I could find. Here are my suggestions:
This is easily the best. Smooth. Writes great. Starts writing immediately. Never leaks. This is easily my most used.
Downside: I like ultrafine line width, this is kinda medium line width
Similar to the above, but slightly thicker line width (if you prefer that).
This is a premium and smooth pen with ultra-fine line width, but that comes with feeling very scratchy on paper. This is the best I could find for ultra fine line width.
No-name ultra cheap Ali Express pens
These are dirt cheap from AliExpress and come in fine and ultra-fine line width. They work well enough. Would probably recommend sticking with these if you want fine line width. Probably not worth spending on the premium option above.
I wish I had small enough hands to use this. Really good pen. Looks premium. Very pocketable. The small size limits it to small size ink cartridges and makes it difficult to use with big hands. You can put the cap on the back to make it a comfortable size, but that the makes it back-heavy.
I've used these pens for over a year. They're still working fine. The only problem has been that if I don't use the pen the ink dries up and it stops working. That only pen that gave me trouble with leaks was the Kakimori pen, and that was only because of temperature changes (leaving it in the car, then taking it indoors to a warm environment.....this leaking went away when I left the pen in an office drawer).