Which is the best weapon against common invaders / unwanted visitors in dwarf fortress?

Assuming you aren't specifically interested in training up dwarves as warriors, the best weapon isn't a weapon, it's a trap. Magma traps are the best. Flood traps and deep pit traps with retractable bridges in your main entrance are pretty good. All of these require extensive setup, though, so you are probably going to want a weapon trap, about 3 to 5 spiked balls, axe blades or circular blades per trap. Spiked wooden balls would be the quickest to set up at first. You can place them along the sides of a wide main entrance tunnel, then make a pit trap and retractable bridge in the middle to give wagons a place to travel when the bridge is extended. Spinning glass blades can cause near instant death for most lightly armored enemies without damaging equipment, and spinning metal blades are good against armor.