There is of course no universal answer but based on my personal experience I'd rank sources today as...
ST Dupont for the most consistently smooth, reliable, forgiving and responsive nibs by far closely followed by Caran d'Ache.
For English made pens the answer is simple; Yard-o-Led.
Among the Japanese makers I would rank them as Platinum, then Sailor with Pilot the most inconsistent of the three.
In German companies I'd rank Graf von Faber Castell at the top, then Montblanc with Pelikan the most inconsistent.
Italy is tough. Montegrappa, Aurora, Ferrari da Varese, Delta and OMAS all offer great nibs these days and I can't really choose between them. Signum is pretty close but not quite as consistently great in my experience. Visconti seems to be the least consistent these days.
But the one company that so far, even with all the ownership changes, continues to amaze me is Sheaffer. With the latest ownership change to AT Cross I have to suggest a wait and see position.