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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When you play, find what you like, are you a sniper, hiding, waiting, for the perfect moment that will turn the tide? Or are you Brute force, hitting heavy with grenades and rocket launchers? Maybe The quick shot, coming in, hidden, avoiding major battles, and take out individually.