The best replacement for ktorrent is deluge.

The best alternatives to ktorrent are rated as following:
★★★★★  deluge
★★★  transmission
★★  qbittorrent
  azureus

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Speaking about ktorrent, one probably shouldn't forget about magnet links.

Other search terms related to ktorrent and their ratings are:
★★★★★ magnet links
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Google Trends

Scale is based on the average worldwide traffic of ktorrent in all years. Click on graph for more.
ktorrent
deluge
transmission