The best replacement for rectangles is squares.

The best alternatives to rectangles are rated as following:
★★★★★  squares
★★  triangles
★  parallelograms

Your alternative for rectangles is not listed? Suggest!
Speaking about rectangles, one probably shouldn't forget about diagonals.

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



Google Trends

Scale is based on the average worldwide traffic of rectangles in all years. Click on graph for more.
rectangles
squares
triangles