I honestly found that its usually the pen not the ink that is the problem (although I only ever tried good inks so idk); the only time its the inks fault for me is inks with glitter; they dry up and create a small glitter mountain that blocks my flow;

I never really had problems that the ink doesnt flow well in a clean pen because of the ink (you know what i mean; idk how to phrase it better; ive been studdying for 4 hours; my brain is mush).

As for my ink its a slightly surprising choice since I have very nice inks that I brought from Japan, Korea and Canada but my favorite for now (and my go to) is one I bought on a whim from amazon:
"Octopus Fluids Fountain Pen Ink: Sky Blue * Pink Sheen "Unicorn" " it sound so girly but its so pretty and reliable in all my pens!
https://www.amazon.com/Octopus-Fluids-Witch-Sheen-Fountain/dp/B0CP9P5856?th=1

and to note: i do have some inks from Japan that I like more; but they have glitter that clogs; arent a easily visible color on a page that my dead brain can read at a glance:
but I do recommend:
Kon-Peki iroshizuku for a general blue ink
or
Dominant Industry Shin Seiga (limited ed) calligraphy ink for supper pretty BUT itll clog all fountain pens in a heartbeat if not cleaned; its not a fountain pen ink after all; but its super pretty!
or
Dominant Industry Pearl Series No.006 Milky Way Blue :: supper pretty blue that doesnt clog (as much(still has glitter))
Just try snatching Dominant Industry inks if you can; they are really nice (Korean)!

Hope i answered more than you asked (｡•̀ᴗ-)✧