The best replacement for trebuchet is catapult.

The best alternatives to trebuchet are rated as following:
★★★★★  catapult
★★  onager
★★  mangonel
★★  verdana
★★  google now launcher
★★  arial
★★  launcher
★  nova
★  tahoma
★  google now
★  ballista
★  launcher 3
★  google launcher
  catapults
  nova launcher
  catapult better
  torsion catapult

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Speaking about trebuchet, one probably shouldn't forget about counterweight.

Other search terms related to trebuchet and their ratings are:
★★★★★ counterweight
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Google Trends

Scale is based on the average worldwide traffic of trebuchet in all years. Click on graph for more.
trebuchet
catapult
onager