The best replacement for icsp is spi.

The best alternatives to icsp are rated as following:
★★★★★  spi
★★★★  isp
★★★  ftdi
★  jtag

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

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



Google Trends

Scale is based on the average worldwide traffic of icsp in all years. Click on graph for more.
icsp
spi
isp