Corto Maltese is the most famous and arguably the best creation of Hugo Pratt, an Italian author (writer and draughtsman) who remains pretty much unknown in the UK.
Personally The Celts is my favourite arc in Corto's adventures, but it is really difficult and unfair to highlight only one in a series that works as a whole.
Although Corto is a lone, romantic (not sentimental!), sort of anti-hero (and when he was created this kind of characters were not as fashionable as they are these days), the series of books is populated by wonderful, wholesome characters that grow as the episodes develop, dropping in and out of storylines, reappearing later to fulfill their roles. Rasputine is the first to come to mind, but especially the female characters are extraordinarily powerful and 'real': This is not your super heroes kind of graphic novel, where women are pneumatic page-fillers, and in this case they pretty much are the rulers of the game.
In terms of writing for graphic novels, i can only think of another major work that stands comparison to this masterpiece: The Sandman, by Neil Gaiman. It may serve as a good reference to the ones that love the complex web of references created by Gaiman, to highlight that Hugo Pratt managed to create a similarly structured universe, long before the Sandman became popular.
Pratt and his alter-ego Corto are less narrative - at times the silences are long and more meaningful than the drawings themselves and throughout the series there are unforgettable moments (see, for exampe, in The Celts, the sequence in which Corto wakes up in Stonehenge). Hystory, politics and beautiful artwork, without the overbearing tones that at times the Sandman contains. (and please don't take me wrong: I actually love the Sandman).
The drawings are beautiful: In Europe they were originally printed in glorious black and white (that's how I got to discover Corto and that's how I still prefer it), with long shadows highlighting the meaningful details of carefully framed "shots".
More recently, updated versions with detailed introductions and sketches, (at times even with essays from the likes of H. Eco) have come out in watercolour and they're also worth collecting.
I am not sure why UK publishers have not yet given the deserved attention to Hugo Pratt, but I guess it has to do with the way most anglo-american audiences still think about this kind of work as comics (as opposed to the latin countries love for the "Bande Dessinee").
I am not sure if fantastic/poetic realism describes the beauty of Corto's wanderings but if ever there was a graphic novel for grown-ups this is it.
All arcs can be read independently so, if you find one in English, give it a try!