For a game that began as a project called Monado: The Beginning, Xenoblade Chronicles does indeed feature the eponymous Monado rather prominently. It's likely one of the most iconic swords out there in gaming now, and Xenoblade did itself change how people design RPG worlds, and RPGs in general.
Though many people know the Monado as it appears on the main menu and in much of the promotional material, Shulk's Monado actually goes through quite a few changes, and his isn't even the only one there is. Though it can be argued that the sequel features Monados of its own, this is going to focus exclusively on those that appear in the original.
7 Meyneth's Monado
The plot of Xenoblade Chronicles advances in ways that might seem quite unexpected based on how it begins, with Shulk's visions hinting at things that may seem obvious in retrospect. One of those is Meyneth who, without many spoilers, is central to the Mechonis and has a Monado of her own.
Contrasting with the smooth curves and soft angles of Shulk's red Monado, Meyneth's Monado is a teal shade along the whole blade, with purple veins running along it. It's hollowed out in the center and pronged open at the tip, while the guard angles down sharply. Just above that are three purple orbs connected by black lines. While wielded by a particular party member, the Monado is split into two, looking almost identical to its combined form.
6 Monado Replicas
Though Shulk's Monado and its evolved forms may be beloved and home to many unique gameplay aspects, they're far from invincible. At a certain point, Shulk is forced to take up replicas of the Monado that can replicate many of the functions of the original, though at significantly reduced power.
There are actually six Monado Replicas you can acquire in the base game, one given to you by default and the rest earned through quests. They have three different models — the first attempts to emulate the Monado's curves though in a much bulkier, mechanical form; the second is a large symmetrical chunk that juts out at the top with metal engraving along them, and the final has the most extreme redesign, featuring a guard along the hilt, fanned wings to the side, and an oversized, thick blade at the center.
5 Monado III
Throughout the many twists and turns of Xenoblade's plot, the Monado evolves with it, in terms of capability and appearance. The Monado III can actually only be used very briefly in the game, unlocked during the very final boss fight of the main game, though can be carried forward into a New Game Plus file.
The Monado III, the final form of Shulk's true Monado, is drastically different from his previous Monado forms. Where the others were angled and smooth overall, with a blade that opened to reveal another blade of pure light, this Monado is itself made of ethereal light. The whole blade is interlaced with it, weaving around itself like a delicate pattern until it combines into the central sigil. Surrounding the sigil are edged arcs, while the handle is smooth, ending in a triangular pommel.
4 Monado Replica EX
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Not present in the original Xenoblade Chronicles, the Monado Replica EX (or REX, if you prefer references to the sequel) appears in the Future Connected story of the Definitive Edition, replacing Shulk's true Monado for what we'll call ethical reasons. It makes sense in the grand scheme of things.
The Replica EX, and its almost identical EX+ form, takes on a form that's a blend of the default replica Monado and the true Monado. It loses much of the smoothness of the original, becoming more mechanical and rugged. That said, it retains the striking red coloration and still opens to reveal the blade of light, with fumes of light somehow shooting out of exhaust pipes on the side. It's a fun blend of the game's core opposites between the Bionis and Mechonis.
3 Monado II
Received after a visit to the rather accurately named Prison Island above Eryth Sea, the Monado II comes with various story revelations, further capabilities to upgrade, and the ability to harm faced Mechon. Oh, and a wonderful new design, of course.
At first glance, the evolved Monado may look quite similar to the base Monado. It's a bit like an arbitrary UI update, moving away from smooth and soft to sleek and edged. It bears a longer blade now, which ends in a sharper point, and the blade-edge side now ends at a lower point, arcing itself past the central sigil. The slots at the core of the original are now removed, replaced with empty space that lets the blade itself open more smoothly.
2 Zanza's Monado
Zanza is rather central to the world of Xenoblade, quite literally, and is the companion figure to Meyneth, each representing the Bionis and Mechonis respectively. Zanza, however, is a much larger threat for reasons the plot will very much have you debate on yourself.
For as serious an antagonist as he is, however, his Monado is something straight out of Kingdom Hearts. It's quite a hard thing to describe. It has the same sigil above the hilt as other Monados, with the central blade fanning out alongside the sigil and curving outwards to a pointed edge at the tip. Of course, it also has spined wings spanning from the center of the blade as well, and spines all around it. It's so ugly, but how extra it is makes it quite enjoyable in a funny, gaudy kind of way.
1 Monado I
The Monado I, the original Monado you wield at the beginning of the game as Dunban, and have shoved in your face in a lab from the opening section of the game, is of course the best. Other designs may be better, other abilities more powerful, but the sheer iconic qualities make it impossible to ignore the original Monado.
Forced to look at it from the very opening screen of the game, the original Monado is a completely smooth blade, its edge itself illogically able to damage things despite being completely smooth. At its opposite side is a sheer straight angle, and the sigil at its center that displays the various Monado arts is clear to see. It's a great look because it's so illogical as a weapon, and that's almost part of its lore and why it has a great beam of light that's revealed to actually do the damage the blunt blade could never.