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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In the first Halo, I'd say the overall best weapon was the Pistol. Good for short and fairly long range, and if you're good enough it only takes three shots to kill someone. In Halo 2, I'm sure my favorite weapon will be the plasma sword, even though you can only get a good swing from like 15-20 feet away or so.