The best replacement for sans serif is serif.

The best alternatives to sans serif are rated as following:
★★★★★  serif
★★★  serif legibility
★★  serif when to use
★★  slab serif
★★  serif fonts
★★  times new roman
★  helvetica
★  arial
  tahoma
  serif font
  microsoft sans serif
  ms sans serif
  san serif

Your alternative for sans serif is not listed? Suggest!
Speaking about sans serif, one probably shouldn't forget about garamond.

Other search terms related to sans serif and their ratings are:
★★★★★ garamond
★★★★★ baskerville
★★★★ bodoni
You have in mind a companion for sans serif and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of sans serif in all years. Click on graph for more.
sans serif
serif
serif legibility