What a very difficult question to answer. There are so many weapons that I've seen, mostly from shmups and RPGs. I was tempted to say a tricked out Yoshitsuna Mk. 1 from Disgaea, but I'll go for the most versatile and cool weapon I can think of...

Temjin's Beam Rifle/Sword from the Virtual On series. (I forget the name of it.)

It has the best of both worlds! A long range beam shot for hitting targets at a distance and the ability to slash at close targets using the sword. The special attacks are also interesting. The first special splits the sword in two and shoots a giant laser, doing nice damage to whatever it hits. The second enlarges the sword and turns it into a surfboard, allowing you to run over your opponent and do a big chunk of damage.

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