The best replacement for metal gear is splinter cell.

The best alternatives to metal gear are rated as following:
★★★★★  splinter cell
★★  uncharted
★  dmc
★  syphon filter
★  deus ex
  infamous
  metal slug
  shagohod
  plastic gear

Your alternative for metal gear is not listed? Suggest!
Speaking about metal gear, one probably shouldn't forget about david hayter.

Other search terms related to metal gear and their ratings are:
★★★★★ david hayter
★★★★★ ps3
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Google Trends

Scale is based on the average worldwide traffic of metal gear in all years. Click on graph for more.
metal gear
splinter cell
uncharted