The best replacement for speedplay is look.

The best alternatives to speedplay are rated as following:
★★★★★  look
★★★  shimano
★★★  spd
★★  dura ace pedals
★★  spd-sl
★★  keo
★  look keo
★  look pedals
★  spd pedals
★  look keo blade
★  bebop
★  time pedals
★  crank brothers
★  eggbeater
  shimano pedals
  keo blade
  spd sl
  ultegra pedals
  keo pedals
  spdsl
  time rxs
  look keo pedals
  shimano dura ace pedals

Your alternative for speedplay is not listed? Suggest!
Speaking about speedplay, one probably shouldn't forget about bont shoes.

Other search terms related to speedplay and their ratings are:
★★★★★ bont shoes
★★★ sidi
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Google Trends

Scale is based on the average worldwide traffic of speedplay in all years. Click on graph for more.
speedplay
look
shimano