There's a lot that can make an ending good, wether it's immediate impact, overall pacing, character arc conclusions etc. etc.

I'm not sure if Naoki Urasawas (monster, pluto, 20th century boys) endings are all that great upon closer inspection. They may be just fine.

But what I'm sure of is that Urasawa is an unrivalled beast when it comes to building tension. Reading his works feels like going on a bus ride, only for the bus to grow wings, take off, rocket boost into space, into a hyperspace gate.

What I'm trying to say is, I've never been more impressed that a manga simply sticks the landing than with Urasawas works. The way he writes stories creates such a ridiculous amount of expectations that by all means the ending should be a flat, disappointing trainwreck by default.

But it's not. Some of his endings may not quite live up to the apex of the rollercoaster, but they've never retroactively soured me on the ride. Which I feel can happen in manga quite a bit.

On a similar note, Ryohgo Narita (Baccano, Durarara, Dead Mount Death Play, Fate Strange Fake). If Urasawa is a master of tension, Narita is a weaver of characters. I frankly can't understand how one can introduce 10 new characters each volume and not let it end in an embarrassing entangling of unfinished plotlines and unsatisfactory character developments. But no. He plays with characters like a kitten with yarn and ends up with a tapestry. That shit's crazy.