The best replacement for tensioner is idler pulley.

The best alternatives to tensioner are rated as following:
★★★★★  idler pulley
★★★★  chain guide
★★★★  pulley
★  tensioner pulley

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Speaking about tensioner, one probably shouldn't forget about timing belt.

Other search terms related to tensioner and their ratings are:
★★★★★ timing belt
★ coaster brake
★ serpentine belt
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tensioner
idler pulley
chain guide