The best replacement for 6x6 is 6x7.

The best alternatives to 6x6 are rated as following:
★★★★★  6x7
★★★★  8x8
★★★★  645
★★★  4x4 fence posts
★★★  4x4
★★  4x4 for deck posts
★  4x4 posts
★  4x5
★  8x8 argo
★  5x5
  argo 8x8
  4x4 deck post
  ranger 4x4
  6x4 5
  argo

Your alternative for 6x6 is not listed? Suggest!
Speaking about 6x6, one probably shouldn't forget about mamiya rb67.

Other search terms related to 6x6 and their ratings are:
★★★★★ mamiya rb67
You have in mind a companion for 6x6 and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of 6x6 in all years. Click on graph for more.
6x6
6x7
8x8