What's a good TR weapon investment (or general neutral harasser weapon) to turn the harasser into a good mobile weapon platform?
It depends. Do you are a lone driver? Are you planing to snipe enemy tanks or Sundis from far away? Use the helbarde. Are you lone and hunting infantry. Use the gatekeeper or run them over. Are you playing with a friend, choose as recommended above. Are you playing with three friends than there is no reason to choose any above the Vulcan