Goodfellas (Martin Scorcese, 1990)
Though it never bored me for real, it lost pace after some time... It's still one of the greatest gangster movies of all time but I wouldn't dare to compare it to Godfather trilogy or Once Upon a Time in America. Those two are light years ahead of anything ever done!
It's in the same league of those two gangster films, in my opinion (I even prefer it to
The Godfather Trilogy, which I also love). It's less operatic and more rock'n roll in style and therefore a bit different than the other two films you mentioned, but quality-wise I think
Goodfellas is definitely up there with
OUATIA and
The Godfather.
The first time I watched the film (I realize this is not your first time), I LOVED it, but (like you) I also thought the first part was superior to the second part of the film. However, oddly enough, on rewatches, the second part became just as satisfying for me as the first. There are few sequence in cinematic history that have a better visual rhythm than the paranoid helicopter sequence. It's sheer brilliance.
I also don't get your issues with the narration (unless you have something radical against narrations in general, of course). It's one of the best and most effective uses of narration in any film. The insight it offers into the main character's mind is absolutely vital to our perception of the film's content.
Anyway, I realize that you like the film, but you should like it even more.