Staff is the best weapon generally for blm in most situations. It is their strongest melee weapon. Staff will eventually give you the weapon skill Spirit Taker, which gives you the same amount of MP that it hits for. From level 51 on, the elemental staves are the best for spell casting. Prior to 51 a blm will find wands (club) useful for the spellcasting stats. Also at higher levels a scythe can be good when farming on very weak mobs.
Short answer is Staff.