Their is no best weapon really,if you had a plasma sword,sure you can kill something instantly with your lunge attack,but if they don't know there could be a sniper with a great head shot,and with a sniper it is really useless when in close range.
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All the weapons was balanced in Halo and hope so in Halo 2 too.Yesit depends on the player not the weapons. I like to rush em with AR and a plasma rifle/needler and with some frag grenades,tasty collection of death spreaders :)