I understand what the poster above says about the Frozen Tuna (sorry I can't remember your name, I hit the reply button before copy/pasting), there's plenty of weapons in Dark Cloud that I like not simply because of sheer attacking power, but because of their aesthetic charm, or the challenges involved in building them up (or not) ;^)
For Toan, my very favorite sword is the Mardan Eins, which ultimately builds up to the Arise Mardan. It's got a pretty good attack power (250), and can only be earned by catching Mardan Garayan fish. On that note, the only way to build it up is by catching those fish, as well. Which, when you think about it, makes it the easiest, and cheapest, weapon to build up to the maximum level. ;^)
The next favorite is the 7th Heaven, just because of the look of it. I tend to look on the Chronicle(s) as pure meat 'n' potato swords to get me out of a jam.
For Xiao, of course, it's Steve/Super Steve. Too bad that (it? he?), doesn't have the maximum attacking power. :^/
For Ruby, I keep rings in several stages of build up, just because of the cool looks some of them have. Obviously, I've got to go with the Secret Armlet as the favorite, but I also keep Satan's Ring, Athena's Armlet, and the Pokehul in a state of constant 'upgrade,' with all the Synthspheres I can attatch so they're a bit stronger. While the Secret Armlet is indeed awesome, it just looks, well...plain. :^\
For Goro, the Frozen Tuna falls in the same comedy/looks category. I like to run through the Divine Beast Cave with it and off monsters occasionally. The favorite is the Inferno, for no reason other than my favorite colors are black and red. ;^)
And Ungaga, well, I wish they'd given him more 'ultimate' options than the Babel's Spear. Still, much time has been spent making Synthspheres out of his Scorpions for all characters. And the Cactus looks cool. But since his weapons go through Repair Powder like mad, it only makes sense that the favorite here is ultimately the Babel's Spear.
And finally, for Osmond, it's the Supernova (his maximum flamethrower-type weapon; pay no attention to the Prima strategy guide that tells you EVERY weapon maxes out to the Star Breaker). What a weird gun! It looks like the old West meets Star Trek or something, here. It's not as powerful as the Star Breaker, but its attack is impressive. It gives even more character to the little flying bunny. ;^)
And I'd go into Dark Cloud 2, but it's been a long time since I've played it, and this post is getting long enough.