Great manga or best manga?
I haven't read it all, but if I had to say, it's the most imaginative manga ever.
Just look at the members of the Straw Hat Pirates.
They gots a big-nosed sniper who tells tall tales, a living skeleton musician with an afro and a sword cane who also happens to be a massive pervert, a cyborg who runs on cola, a reindeer who ate a fruit that magically personified him who is also a trained medical doctor, a rubber man, a hot navigator chick who fights with a weather-powered staff, a swordsman who fights with three swords (one in his mouth, naturally,) an expert short-order cook with French-sensibilities and amazing kickboxing skills, and a hot archaeologist chick with the power of infinite arms of any size.
With a protagonist crew like that, how can the manga/anime not be awesome?